Tribochemistry, mechanical alloying, mechanochemistry: what is in a name?

Emmerling, Franziska

doi:10.3389/fchem.2021.685789

Tribochemistry, mechanical alloying, mechanochemistry: what is in a name?

Emmerling, Franziska

Publisher: Zenodo
DOI: 10.3389/fchem.2021.685789
Repository: zenodo.org
Language: en

Abstract

Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.

Full text

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name? Adam A. L. Michalchuk 1 *, Elena V. Boldyreva 2 , 3 *, Ana M. Belenguer 4 , Franziska Emmerling 1 and Vladimir V. Boldyrev 2 , 5 1 Federal Institute for Materials Research and Testing (BAM), Berlin, Germany, 2 Novosibirsk State University, Novosibirsk, Russia, 3 Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia, 4 Yusef Hamied Department of Chemistry, University of Cambridge, Cambridge, United Kingdom, 5 Voevodski Institute of Chemical Kinetics and Combustion SB RAS, Novosibirsk, Russia Over the decades, the application of mechanical force to inﬂuence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the ﬁeld. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be deﬁned to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and selfexplanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community. Keywords: mechanochemistry, tribochemistry, mechanical alloying, tribology, mechanical activation, nomenclature, mechanochemical pictographs INTRODUCTION Chemical transformations initiated by mechanical energy appear to be the ﬁrst reactions that humans learned to induce and control, even before thermal reactions were possible. In fact, the ﬁrst combustion reactions were produced through mechanical action: by percussion or by friction, i.e. they were mechanochemical or tribochemical, if modern terminology were used. Throughout human history, mechanically induced chemical reactions have accompanied many signiﬁcant technological advances. For example, since the discovery of black powder in ca. 220 B.C.E., explosives have allowed the advent of mining and have facilitated the construction of cities and infrastructures. More recently, the continued development of mechanochemistry promises to revolutionize the chemical industry, providing synthetic routes devoid of environmentally harmful solvents (James et al., 2012; Edited by: Lev S. Rapoport, Holon Institute of Technology, Israel Reviewed by: Kenneth John MacKenzie, MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand Vladislav V. Gurzhiy, Saint Petersburg State University, Russia *Correspondence: Elena V. Boldyreva [email protected] [email protected] Adam A. L. Michalchuk [email protected] Specialty section: This article was submitted to Solid State Chemistry, a section of the journal Frontiers in Chemistry Received: 25 March 2021 Accepted: 03 May 2021 Published: 26 May 2021 Citation: Michalchuk AAL, Boldyreva EV, Belenguer AM, Emmerling F and Boldyrev VV (2021) Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name? Front. Chem. 9:685789. doi: 10.3389/fchem.2021.685789 Abbreviations: ACN, Acetonitrile; AFM, Atomic Force Microscopy; BAM, Bundesanstalt für Materialforschung und -prüfung; BUMS, Bottom Up Mechanochemical Synthesis; CIF, Crystallographic Information File; DAC, Diamond Anvil Cell; DBU, 1,8Diazabicyclo[5.4.0]undec-7-ene; DDQ, 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone; FAIR, Findable, Accessible, Interoperable, Reusable; FM-PES, Force-Modiﬁed Potential Energy Surface; GPa, Giga Pascal; IA-LAG, Inadvertent Liquid Assisted Grinding; IUPAC, International Union of Pure and Applied Chemistry; ILAG, Ionic liquid assisted grinding; IMA, International Mechanochemical Association; LAG, Liquid Assisted Grinding; LVP, Large Volume Press; MOF, Metal-Organic Framework; NG, Neat Grinding; OAD, Oxalic acid dihydrate; PES, Potential Energy Surface; POLAG, Polymer Assisted Grinding; PVP, Polyvinylpyrrolidone; RAM, Resonant Acoustic Mixing or Resonant Acoustic Mixer; RPM, Revolutions per minute; TSE, Twin Screw Extrusion. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857891 REVIEW published: 26 May 2021 doi: 10.3389/fchem.2021.685789 Baláˇ z et al., 2013). The potential for mechanochemistry to have paradigm-changing impact across the chemical sciences has placed the ﬁeld amongst IUPAC’s‘10 chemical innovations that will change our world’(Gomollón-Bel, 2019). The earliest written record of a mechanochemical transformation seems to be that by Theophrastus of Eresus, in his book “On Stones”of ca. 315 B.C.E (Takacs, 2000). Theophrastus describes the reduction of cinnabar to mercury through grinding using a copper mortar and pestle. Although grinding and milling were used extensively over the centuries for the processing of grains, minerals, and even pharmaceuticals, mention of mechanochemical processes in scientiﬁc literature did not reappear until the 19th century. These early reports include those by Faraday (1820) on the dehydration of crystal hydrates (Heinicke, 1984;Takacs, 2013), Carey-Lee (1866) on the decomposition of silver, gold, and mercury halides on grinding (Carey-Lea, 1892;Carey-Lea, 1894), and by both Ling and Baker (1893) (Heinicke, 1984;Takacs, 2013) and Flavitsky (Flavitsky, 1902;Flavitsky, 1909) who described organic chemical reactions upon grinding. The attention of mechanochemical investigation soon expanded to a wide range of material types, and explored an array of phenomena including the initiation of explosives by impact and friction (Bowden et al., 1947;Bowden and Gurton, 1949;Fox, 1975;Aduev et al., 1999), and the mechanical decomposition of polymers (Butyagin, 1971;Polukhina and Baramboim, 1975;Oprea, 1979;Sohma, 1989;Delogu et al., 2017). Similarly, mechanochemical investigation into areas including the chemical processes accompanying mining, metallurgy, and the manufacturing of various oxide and chalcogenide materials became a prominent direction of research (Senna, 1993;Butyagin, 1994;Boldyrev, 1996; Fernández-Bertran, 1999;Steinike and Tkáˇ cová, 2000;Senna, 2001;Epelak, 2002;Boldyrev, 2006;Buyanov et al., 2009;ˇ Sepelák et al., 2012;ˇ Sepelák et al., 2013), expanding toward the preparation and processing of ﬁne chemicals and pharmaceuticals (Baba et al., 1990;Kuzuya et al., 1991;Otsuka et al., 1994;Dubinskaya, 1999;Kondo, 2000;Boldyrev, 2004; Otsuka et al., 2011). The 20 th century represents a period of remarkable development of the fundamental aspects of mechanochemistry and of signiﬁcant progress in scaling mechanochemical reactions toward real-world industrial applications. Although progress in mechanochemistry through the 20th century was dominated by studies of metals, inorganic compounds, materials, and catalysts, signiﬁcant advances were also made in the mechanochemistry of organic polymers and drug compounds and formulations (Boldyrev and Avvakumov, 1967;Butyagin, 1994;Boldyrev, 1996;Fernández-Bertran, 1999; Steinike and Tkáˇ cová, 2000;Watanabe et al., 2001;Watanabe et al., 2002;Watanabe et al., 2003;Boldyrev, 2004;Boldyrev, 2006; Buyanov et al., 2009). A number of dedicated texts on the historical development of mechanochemistry are available elsewhere (Boldyrev and Tkáˇ cová, 2000;Takacs, 2013; Boldyrev, 2018). To date, mechanochemical approaches being applied to transformations from across the chemical sciences have been reported, spanning from the synthesis of inorganic and organic compounds [including those as complex as peptides (Hernández et al., 2017;Maurin et al., 2017)] through to the preparation of large porous frameworks such as metal-organic frameworks [MOFs(Stolar et al., 2017;Stolar and Uˇ zarevi´ c, 2020;Wang et al., 2020;Zhou et al., 2020;Głowniak et al., 2021;Stolar et al., 2021)]. Moreover, the scale of mechanochemical reactions has ranged from the mechanical manipulation of single atoms and molecules (predominantly, synthetic and natural polymers) using atomic force microscopy (Kaupp, 2009;Ribas-Arino and Marx, 2012;Makarov, 2016;Li et al., 2017) to the induction of reactions in multi-component inorganic and organic powder mixtures in ball milling reactors or extruders (Iwasaki et al., 2010;Am Ende et al., 2014;Oliveira et al., 2017; Andersen and Mack, 2018b;Egleston et al., 2020). Alongside synthetic covalent chemical reactions, a wide range of supramolecular assemblies have been also prepared by mechanical treatment, including cocrystals and salts (Myz et al., 2009;Weyna et al., 2009;Friˇ sˇ ci´ c, 2012;Myz et al., 2012; Braga et al., 2013;Hasa et al., 2015), as well as non-covalently bound mechano-composites such as drug delivery devices comprising active pharmaceutical ingredients with excipients (Shakhtshneider et al., 2007;Shakhtshneider et al., 2014a; Shakhtshneider et al., 2014b;Lomovsky et al., 2017;Ogienko et al., 2018;Bychkov et al., 2019;Adekenov et al., 2020;Skripkina et al., 2020). Moreover, many organic mechanochemical syntheses have been successfully scaled-up (Iwasaki et al., 2010;Am Ende et al., 2014;Crawford et al., 2017;Troﬁmova et al., 2018;Stolar et al., 2019;Baláˇ z et al., 2020;Crawford et al., 2020;He et al., 2020;Titi et al., 2020;Stolar et al., 2021), offering a direct route to translate mechanochemical synthesis toward industrial scale applications. It can be therefore expected that many industrial syntheses will be soon conducted mechanochemically, alongside the long-standing technologies of inorganic mechanosynthesis and of powder processing e.g. in the pharmaceutical, pulp-paper, mining, explosives, and food processing industries. Indeed, this enormous range of applications for mechanochemical preparations demands that the processes which govern their transformation must be equally diverse, as shown in the hierarchical diagram in Figure 1. There is some elegance to this complexity: many of the processes which govern mechanochemical reactions of complex systems can be largely deconstructed into some combination of the elementary processes which occur in simpler systems. For example, mechanical treatment of a single powder particle will still involve geometric distortion of its molecular substituents (Haruta et al., 2019), and there remains the potential for molecular or atomic electronic excitation/emission processes to occur. This behavior is clear for example in high pressure experiments of molecular solids, wherein mechanical action of the bulk lattice yields geometric (Fabbiani et al., 2005) and electronic distortions (Pore ˛ba et al., 2019) or excitations (Tulip and Bates, 2009) at the molecular or atomic level(Boldyreva, 2019; Katrusiak, 2019;Zakharov and Boldyreva, 2019). The dynamical stressing (compression or shearing) of solids can also cause chemical species within the solid state to approach each other at high velocities, akin to molecular collisions in ﬂuids. Such dynamical interactions can occur either within a single particle Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857892 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry (Landerville et al., 2009;Zhang et al., 2015;Steele et al., 2020), or at inter-particle contacts (Ferguson et al., 2019). These solid state “molecular collisions”have been suggested as the origin of slipinduced “hot spots”(Zhang et al., 2008;Zhang et al., 2015;Zhang et al., 2018), or even covalent bond formation (Engelke and Blais, 1994). That said, with increasing complexity of the system, many more and new pathways exist by which mechanical action can exert inﬂuence. This hierarchical phenomenology is again exempliﬁed by the study of high-pressure phenomena in molecular solids. Mechanical force not only affects molecular geometry but can also inﬂuence the intermolecular non-covalent interactions, leading to changes in crystal packing (polymorphism). By manipulating crystal packing, mechanical force thus offers a route to modify bulk physical properties such as lattice stability, melting temperatures, and compressibility. Thus, although many of the elementary stages of a solution-based chemical reaction may still apply to mechanochemical reactions, many additional elementary stages must be also considered to fully account for the mechanism of mechanochemical transformations. Regardless of the exact elementary processes involved, mechanical action drives the system out of equilibrium, which may involve a classical phase transition (Drebushchak et al., 2011)orvia some other transient metastable state (Butyagin, 1971). This intermediate state can endure e.g. under continuous mechanical action and provide modiﬁed tribological properties (Prentice et al., 2020;Reddyhoff et al., 2021), or relax when the mechanical perturbation is released to yield a reaction product (Boldyrev, 2006). Owing to the signiﬁcant kinetic barriers in solid state reactions, this reaction product may not be always the most thermodynamically stable one (i.e. the global minimum of the system), but it will always be the most accessible product under the given mechanochemical conditions (Lin and Nadiv, 1979). Identifying, understanding, predicting, and ultimately FIGURE 1 | Hierarchical representation of the major effects (gray) of mechanical action on different systems, ranging from single molecules to multi-phase solid powder mixtures (gold). Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857893 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry controlling these pathways leading to the mechanical manipulation of matter is crucial, should mechanochemical approaches ever become equally controllable as the welldeveloped aspects of solution and gas-phase reactivity. Deconvolution of these complex phenomena requires the use of a common language which allows the effective communication of the process being discussed. Only in this way can we hope for a concerted and coherent effort towards elucidating mechanochemical reaction mechanisms and driving forces, and therefore gain control over these reactions to make them possible to reproduce and scale. Despite decades of mechanochemical research, the need to agree on using certain terms, on how to deﬁne accurately and unambiguously the experimental procedures, and how to present the results have been not widely seen as necessary until very recently. For almost a century, the mechanochemical community remained relatively small, although it covered a diverse range of ﬁelds. Researchers knew not only the scientiﬁc research of the others but often knew each other personally. The basics of mechanochemistry, as well as the experimental and computational protocols were discussed regularly in original publications, and at many dedicated mechanochemical seminars and conferences. The developments in the ﬁeld were regularly summarized in thoroughly detailed monographs and reviews, that have now become seminal (Thießen, 1965;Avvakumov, 1972;Boldyrev, 1972;Boldyrev, 1983;Heinicke, 1984;Tkáˇ cová, 1989; Avvakumov et al., 2001;Suryanarayana et al., 2001;Delogu et al., 2004a;Boldyrev, 2006;Butyagin, 2006;Zyryanov, 2008; Buyanov et al., 2009). The foundation of the International Mechanochemical Association (IMA) (International Mechanochemical Association, 2020) under the guide of IUPAC in 1989, was an important event that marked the formation of a mature scientiﬁc community with a common language and a clear vision of the scientiﬁcﬁeld. As an increasing number of groups have begun in recent years to enter the ﬁeld of mechanochemistry independently of IMA, the quickly growing community has since become scientiﬁcally heterogeneous. In contrast to its original composition, the mechanochemical community is now becoming enriched with researchers from very different backgrounds and expertize, many of them being originally experts not in the solid-state, but in solution-based chemistry. This diversiﬁcation in its membership has brought with it many new and exciting research challenges, leading to a much greater impact of mechanochemistry than ever before. Yet one cannot ignore the fact that with diversiﬁcation of the community comes an expanding breadth of specialized scientiﬁc languages. As a consequence, the heterogenous community may not always understand each other effectively, or may become increasingly unaware of the mechanochemical knowledge that has been accumulated in early publications, that itself can be perceived as being written in “another scientiﬁc language”. There is the real danger that the lack of use of a common scientiﬁc language can prevent the community from meeting the challenge of constructing the “Tower of Mechanochemistry”, as it did millennia ago in relation to the Tower of Babel. WHAT IS IN A NAME? Anthropologists argue that humankind evolved due to our capacity to conceive abstract phenomena and communicate these phenomena through complexity of language. In this light, it is no wonder that philosophers have attached such signiﬁcance to the selection and connotation of words. In Plato’s famous dialogue Cratylus, he argues: “a name is an instrument of teaching and of distinguishing natures, as the shuttle is of distinguishing the threads of the web”(Plato, 1961). The chemical sciences have followed true to Plato’s logic. Chemical reactions are denoted according to the type of energy used to initiate a chemical reaction, their nature is revealed through their name: thermo-chemistry, electro-chemistry, magneto-chemistry, photo-chemistry, and radiation-chemistry. Thus, adequately naming a chemical reaction requires an elementary understanding of the underlying chemical and physical processes. In the early 20 th century, Ostwald (Ostwald, 1919) noted reports that existing nomenclature in the chemical sciences did not fully represent the true nature of all observed chemical reactions, for Carey-Lea demonstrated a unique outcome of thermaland mechanochemical reactions in metal halides (Carey-Lea, 1892;Carey-Lea, 1894). Correspondingly, Ostwald introduced in his 1919 textbook the term mechanochemistry to describe reactions in any state of aggregation which are initiated by mechanical force (impact and friction). A more speciﬁc term—tribochemistry—was subsequently proposed to denote only those chemical and physico-chemical changes which occur in solids in response to mechanical energy (Heinicke, 1970). The term tribochemistry was used widely throughout the 20 th century in relation to processes that occur on grinding, ball milling, comminution, friction, wear, rubbing, and lubrication of solids. With growing diversity of tribochemical reactions, daughter terms became commonplace to facilitate more accurately the communication of the scientiﬁc work. These words included tribocatalysis, triboelectrochemistry, tribosorption, tribodiffusion, tribocorrosion, tribotechnology, tribomechanics, tribogalvanics, and tribometallurgy, each introducing speciﬁc subﬁelds of tribology, the science uniting tribochemistry and tribophysics (Thiessen et al., 1966;Thiessen, 1974;Heinicke, 1984;Holmberg and Erdemir, 2017). A primary aim for introducing these “tribo”terms in addition to Ostwald’s term “mechanochemistry”, was to separate solid-state mechanochemistry from mechanically induced processes that occur in single molecules or liquids. In this way, physical phenomena induced by mechanical action in solids or at their surfaces, including phenomena like mechanical mixing and comminution, were to be denoted with a “tribo”preﬁx. All other mechanically induced phenomena were instead to be deﬁned by a preﬁx“mechano”. Despite growing popularity of tribological nomenclature, analogous terms preﬁxed by “mechano”remained in the literature as synonyms also for processes involving solids, including: mechanochemistry, mechanocatalysis, mechanocorrosion, and mechanotechnology. Moreover, the term mechanical alloying was introduced to deﬁne the process Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857894 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry of forming intermetallic compounds and alloys by mechanical treatment of solid components, many of which could not be accessed by any other way than mechanical treatment (Suryanarayana et al., 2001;Benjamin and Volin, 1974;Koch, 1989;El-Eskandarany, 2020). Mechanical alloying processes are of great practical importance, which may account for the extensive publications of mechanical alloying studies and, consequently, of the very wide usage of this term in the scientiﬁc literature. The nomenclature regarding the mechanical manipulation of single molecules has remained much simpler. Only terms preﬁxed by “mechano”have been commonly used when discussing transformations of singlemolecules induced by mechanical stretching of bonds, e.g. using an AFM cantilever (Kaupp, 2009;Ribas-Arino and Marx, 2012;Makarov, 2016;De Bo, 2018;Zhang et al., 2017; Beyer and Clausen-Schaumann, 2005), or when investigating biochemical and biophysical processes (Keller and Bustamante, 2000;Kushwaha and Peterman, 2020;Boocock et al., 2021). Figure 2 gives an idea of the relative frequencies of how the usage of different terms has changed with time. Tribology—the term introduced by Jost (Jost, 1990) in the 1960s—remains by far the most popular term to date, while mechanochemistry is much less used than tribology,ormechanical alloying. While the organic chemistry community appears to favor the term mechanochemistry, the terms tribology and tribochemistry are more popular amongst chemical engineers and the materials science community (Huq et al., 2020;Moshkovich et al., 2020; Moshkovich and Rapoport, 2020;Rosenkranz et al., 2020; Rosenkranz et al., 2021). In addition, the term mechanical activation is widely used in relation to thermal reactions that are facilitated by mechanical pre-treatment (Pavljukhin et al., 1983;Pavlyukhin et al., 1984;Boldyrev, 1996;ˇ Sepelák et al., 1996; Boldyrev, 1998;Boldyrev, 2006;Kumar et al., 2019;Singla et al., 2020). What is in a name? That which we call a rose by any other name would smell as sweet. This famous phrase by W. Shakespeare serves an important lesson for science: the physical world does not depend on our description of it. Of course, our choice of nomenclature has no inﬂuence on the physical reality of chemical reactions. This nomenclature does, however, inﬂuence on our understanding, communication, and formulation of scientiﬁc principles. Nature is indifferent to our terminology. Scientists, in contrast, are prisoners to nomenclature. Without consistent and precise deﬁnition of scientiﬁc concepts, “the rose”may not smell as sweet. Discussions of nomenclature in science has a long history. Most famous, perhaps, are the classiﬁcations of species proposed by Charles Darwin. The taxonomic classiﬁcation of life demonstrates Plato’sreﬂection of a name in understanding the properties and connections between entities. Similar ontological classiﬁcations have been popular in chemistry throughout its history. For example, chemists routinely classify interatomic interactions according to an abstract deﬁnition of bond order: single,double,triple, etc. This precise nomenclature allows scientists to directly and unambiguously describe a characteristic of a molecule directly by the type of bond. Recently, a standard set of bond descriptions were suggested to deﬁne particular types of molecular interactions (Arunan et al., 2011;Desiraju et al., 2013;Aakeroy et al., 2019). Standardizing nomenclature has long been the focus of the IUPAC. More broadly, ensuring that well-deﬁned and well-classiﬁed ontologies exist throughout the sciences is becoming increasingly recognized as the route to ensure Findable, Accessible, Interoperable, and Reusable (FAIR) scientiﬁc data (Hall and McMahon, 2016;Wilkinson et al., 2016). So, what is in a name? Would that which we call “mechanochemistry”by any other name behave the same? Of course, the physical principles which govern mechanochemical reactions will behave independent of our chosen nomenclature, but will the conceptual constructions we use to rationalize, discuss, and direct scientiﬁc research be so resilient? The idea of complementing the term mechanochemistry by the term FIGURE 2 | The number of papers in Scopus each year in which the terms related to chemical reactions of solids induced by mechanical treatment are used in the title, abstract or as keywords. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857895 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry tribology (tribochemistry + tribophysics), to focus more on transformations involving solids (Jost, 1990), is clear and justiﬁed. In practice, the more general term mechanochemistry appears to survive. Moreover, it is increasingly used as a complete synonym of tribology, also when describing ball milling, grinding, and friction (Thiessen, 1986). This hazy nomenclature would not create much problem if it were always straightforward to identify which mechanically induced physical processes in solids were responsible for the chemical processes of bond cleavage and formation. This, however, is not the case. Deﬁning accurately by its name the nature of a mechanically induced transformation has serious implications for the type and importance of physical processes which must be considered when seeking to understand mechanochemical transformations. Moreover, it is critically important to consider that molecules in the solid state cannot immediately react with one-another. Instead, some solid-state phenomenon must ﬁrst occur which allows collisions not at the level of particles but at the level of molecules and atoms. The nature of this preliminary phenomenon depends on whether the reaction is mechano or tribochemical. For example, whereas mixing and comminution may be dominant preliminary phenomena in mechanochemical reactions, electrostatic charging or generation of defects certainly dominate many tribochemical reactions (Boldyrev, 2006). Hence, focus on chemical equilibria presented in terms of solution chemistry, where “one molecule transforms into another molecule”, are grossly oversimpliﬁed and neglect many of the critical physical phenomena which separates mechano/tribochemistry from solution chemistry. Important phenomena such as triboelectric charging (Matsusaka et al., 2010;Cezan et al., 2019) and the mechanical generation of exposed surfaces (Belenguer et al., 2016;SchneiderRauber et al., 2021) or defects are equally likely to occur in inorganic, organic, and polymeric compounds. It is probable that such phenomena involving defect formation (Boldyrev, 1973; Yelsukov et al., 2013) play a central role in most solid-state mechanochemical transformations, even if they are often overlooked and subsumed by explanations of “mere mixing”. Mechanically generated defects can range from radicals and the isomerization of molecules to extended stacking faults, dislocations, and the formation of shear-induced structures (e.g. shear bands). For example, it is known that grinding initially leads to particle size reduction (Lampronti et al., 2021), and ultimately (after a critical grinding or comminution limit is achieved) to the accumulation of defects within the solid structure (Boldyrev et al., 1996). This change in stress relaxation mechanism can lead to deep mechanical activation thereby greatly affecting the reactivity of the solid. Such effects have been suggested as being responsible for the extended induction periods observed in some mechanochemical reactions (Belenguer et al., 2019b). Mechanical treatment of a solid can also lead to the formation of mesophases, wherein superstructures within the phase become disordered to different degrees. Although this phenomenon appears to be more common in organic solids (Rybin et al., 2014;Descamps and Willart, 2016b;Shalaev et al., 2016;Rybin et al., 2019), it draws analogies to the mechanochemistry of inorganic compounds as well. For example, mechanical treatment can induce disordering of different sublattices such as the disordering or re-ordering of metal cations between octahedral and tetrahedral positions in spinels (Pavlyukhin et al., 1984;Tkáˇ cová et al., 1996;ˇ Sepelák et al., 1996;Sepelak et al., 1997;ˇ Sepelák et al., 2007;Harris and ˇ Sepelák, 2018), or of Al and Si in aluminosilicates (Mackenzie et al., 2000). These mesophases can accompany the step-wize disordering, FIGURE 3 | The potential origins of ﬂuid phases in mechanochemical systems can be broadly classiﬁed as intrinsic and extrinsic, depending on whether they initiate within the solid phases or not. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857896 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry amorphization, and overall structural transformations in solids exposed to mechanical action. Why then is the term mechanochemistry gaining more and more popularity? An intrinsic problem with applying the term tribochemistry in the speciﬁc sense as originally proposed—as opposed to the previously existing term mechanochemistry—is that we must know the mechanism of the process. Speciﬁcally, the term tribochemistry should only be used if it is truly a solid-state reaction. This is not obvious, especially for organic compounds, even if we start with solids (Rothenberg et al., 2001;Tumanov et al., 2017). In many cases, the transformation itself, including chemical synthesis, likely occurs in a ﬂuid phase; the possible origin of this ﬂuid phase can be diverse (Figure 3). This is quite often the case for ball milling, grinding in a mortar, or processing in an extruder a mixture of solid organic compounds (Boldyreva, 2013). Generally, the origin of this ﬂuid phase can be classiﬁed as being intrinsic or extrinsic to the reacting system itself. Intrinsic origins include melting (or contact melting) (Saratovkin and Savintcev, 1941;Gerasimov and Boldyrev, 1996;Urakaev and Boldyrev, 2000a;Boldyrev and Tkáˇ cová, 2000;Chadwick et al., 2007;Michalchuk et al., 2014;Humphry-Baker et al., 2016; Fandiño et al., 2020;Haneef and Chadha, 2020), sublimation of solids (Kuroda et al., 2004;Mikhailenko et al., 2004), or dehydration/desolvation (Losev and Boldyreva, 2014) which result from the excess heating of mechanical impacts or bulk heating during mechanical treatment. Where a solids’glass transition temperature is above the milling temperature, one can consider also the formation of transient amorphous phases (Descamps and Willart, 2016a). It has become very common to explicitly add liquid to a powder mixture to facilitate mechanochemical transformations, a process dubbed liquid assisted grinding (LAG) (Bowmaker et al., 2009; Bowmaker, 2013;Sarmah et al., 2019). This is an obvious extrinsic origin of a ﬂuid phase. Even where researchers do not explicitly add liquid, the powder may “grab”liquid from the environment in a process dubbed inadvertent liquid assisted grinding (IA-LAG) (Tumanov et al., 2017). Although the exact role of ﬂuid phases in reactions that are assumed to occur in the solid state is not yet fully understood, various roles can be considered. The added ﬂuid phase can certainly inﬂuence the mobility of material (by improving rheology, or by completely transferring the process into a solution or a melt) (Boldyreva, 2013;Lapshin et al., 2021). Additional to its inﬂuence on material mobility, melting can also drive erosion in microparticle impact (Hassani-Gangaraj et al., 2018), or can hinder impact-induced adhesion (HassaniGangaraj et al., 2017;Lapshin et al., 2021). A ﬂuid, irrespective of its origin, can create hydrothermal conditions (Temuujin et al., 1998a;Boldyrev, 2002), can modify the mechanical properties of the solids via the Ioffe (Ioffe, 1936), Roscoe (Roscoe, 1934), or Rehbinder (Rehbinder, 1947) effects (i.e. the altering of bulk mechanical properties through surface modiﬁcation) (Zhang et al., 2020;Dang et al., 2021), may inﬂuence triboelectric phenomena (Matsusaka et al., 2010), and can alter the relative stability of product phases through the selective stabilization of surfaces (Belenguer et al., 2018;Belenguer et al., 2019a). In many of these cases the process can and should be classiﬁed as tribochemical. However, where the process in fact occurs in the ﬂuid phase, it can be no longer classiﬁed as tribochemical, nor does it unambiguously qualify to be denoted as a “dry” mechanochemical reaction. By considering the possible intrinsic origins of ﬂuid phases it becomes clear that some compounds are more likely to give rise to tribochemical transformations. For example, solids with high melting temperatures (primarily inorganic solids) will not melt or sublime during ball milling and therefore will most likely react tribochemically. In contrast, materials with low melting temperatures (e.g. most organic or coordination compounds) which are likely to melt during or as a result of mechanical treatment cannot be even strictly classiﬁed as mechanochemical, though tribochemical effects (e.g. electrostatic charging) may still be of importance at the elementary (molecular) level (Bowden and Gurton, 1949;Kajdas, 2013). This represents a critical difference between inorganic and organic “solid state” transformations under mechanochemical action (Boldyreva, 2013). It follows from the above discussion that a reaction in a mechanochemical reactor that starts with solid reactants may not in fact be mechanochemical or tribochemical at all. This is the case when the chemical or physical transformation itself is not related directly to the absorption of the mechanical energy input. For volatile compounds, the solid reactants do not even require physical contact and can remain separated in space (Kuroda et al., 2004). In this particular case, the reaction is, strictly speaking, neither mechano-, nor tribochemical. In most reactions, however, no visible transformation is observed unless the compounds are treated in a mechanical apparatus. Yet, in many such cases the reaction is not mechanochemical, but thermal in nature; the role of the mechanical processing is limited to facilitating the mobility of the solid reactants, bringing them into contact with each other and/or by removing the solid products which are formed at the surface of the powder particles. Such processes are largely responsible for the success of new mechanochemical reactors such as the Resonant Acoustic Mixer (RAM) (Anderson et al., 2014;Michalchuk et al., 2018a;Titi et al., 2020). One could term such reactions as “mixing assisted thermal reactions”, as opposed to mechanochemical reactions. This can be taken as an analogue of stirred solution-phase reactions, wherein stirring does not cause the reaction, but simply facilitates the thermal reaction by driving mass transport. These “mixing assisted thermal reactions”differ from “classical”thermal reactions in a few critical ways. “Classical” thermal reactions in solid mixtures (e.g. in high temperature solid-state synthesis) involve the heating of pre-mixed powders, wherein the powder remains largely unperturbed during the reaction. In contrast, reactions of thermal origin that occur during mechanical treatment are accompanied by dynamically changing local compositions (e.g. from mixing) and ﬂuctuations in particle stress regimes. The mixing can be accompanied by the reduction of particle size (comminution), or changes in their agglomeration state. Together, these phenomena of macroscopic motion of particles, comminution or agglomeration, and the potential for strain-induced dissolution of one phase into another (Vasil’ev et al., 2006;Vasil’ev et al., 2009;Vasil’ev and Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857897 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry Lomaev, 2011;Vasil’ev, 2012;Butyagin, 1984;Butyagin, 2005) yields intimate mixing across lengths of scale. One must also keep in mind that many solid + solid reactions are exothermic, since no entropy is gained during the reaction. Correspondingly, if the mechanical treatment is itself accompanied by signiﬁcant heat evolution, mechanically initiated self-sustaining thermal processes become possible (Takacs, 1998;Bakhshai et al., 2002; Takacs, 2002;Delogu et al., 2004b;Maglia et al., 2004;Delogu and Takacs, 2014;Humphry-Baker et al., 2016). It follows that when mechanical mixing results in a chemical transformation, it is not clear a priori if the mechanical treatment of particles themselves plays a signiﬁcant role, or if the particles are “merely brought into contact”. Regardless, any “mixing assisted thermal reaction”will be to a large extent governed by macrokinetics, i.e. heat and mass transfer processes (Urakaev and Boldyrev, 2000a;Urakaev and Boldyrev, 2000b;Delogu et al., 2003;Lapshin et al., 2021). This thermodynamic feature—that solid + solid reactions are exothermic—raises an interesting question: do all mechanochemical reactions in fact have thermal mechanisms? When mechanical energy (U) is exerted on a system, some of the energy is absorbed by the material (e.g. as work, w) and the rest is released as heat (q): U q + w. Using this rule for the conservation of energy, the amount of energy absorbed by the mechanically treated powder has been approximated by measuring the bulk temperature of the reaction vessel (Butyagin, 1967). What energy is then responsible for the transformation, the work or the heat? It is in fact the answer to this very question that distinguishes a mechanochemical transformation from a thermal transformation. Where the absorbed mechanical energy is itself responsible for the transformation (i.e. a mechanochemical reaction), the surrounding temperature has only indirect inﬂuence on the reaction. Once the transformation has occurred, excess energy is emitted from the system also as heat. Where a new product forms, for example in a multi-phase reaction, this can often take the form of exothermic nucleation. Although proving whether a reaction is thermo or mechanochemical can be challenging, a number of clear examples can be given, including the dark dimerization of aromatic compounds at high pressures (Engelke and Blais, 1994;Politov et al., 2010;Friedrich et al., 2020), the classic examples of the decomposition of metal halides by Carey Lea (Carey-Lea, 1892;Carey-Lea, 1894), the decomposition of alkali metal nitrates, bromates, and chlorates, which give different products and follows inverted trends upon heating and mechanical action (Boldyrev, 1972; Boldyrev et al., 1972;Urakaev et al., 1977;Boldyrev and Heinicke, 1979), or oxidation of gold by carbon dioxide (Thiessen et al., 1970). The relations between mechanochemical and thermochemical nature of the reactions can be compared with a situation when light and temperature act on a sample simultaneously. One can imagine a thermochemical reaction conducted in the presence of light: the mechanism does not become photochemical simply because light is present, it remains thermochemical. On the other hand, during a photochemical reaction, wherein the photon is absorbed, the reaction occurs on an excited state potential energy surface, and following relaxation, excess vibrational energy is emitted as heat. The heat is simply a byproduct of the photochemical transformation; one would not call such a reaction thermochemical. It is therefore clear that many so-called “mechanochemical reactions”are not encompassed by the current IUPAC deﬁnition, which states that a mechanochemical process is “a chemical reaction that is induced by the direct absorption of mechanical energy”(Mechano-Chemical Reaction, 2009). Such distinctions, although semantic, play an important role when considering the types of physical phenomena which may play a role in driving the observed reaction. For example, effects of adiabatic compression, or the generation of vibronically excited states are unlikely to play a role in “mixing-assisted thermal reactions.”On the other hand, slow nucleation and crystal growth—which presumably dominate such thermal reactions—are very different from the fast cooperative interfacial propagation processes which can be expected for “true”mechanochemical reactions in which mechanical energy is directly transferred into high-level vibrational or electronic excitations (Coffey and Toton, 1982; Dlott and Fayer, 1990;Tokmakoff et al., 1993;Luty et al., 2002; Eckhardt, 2006;Michalchuk et al., 2018b;Michalchuk et al., 2019a;Michalchuk et al., 2021). Selecting a proper term for a process involving solids that occurs in response to mechanical action is thus intrinsically challenging; the correct term can be only given after the mechanism for the reaction has been established. Even for ﬂuids, deciding on the term “tribochemistry”or “mechanochemistry”may be not always straightforward. Not only can mechanical treatment convert solids to ﬂuids, but it can also convert ﬂuids to solids. Shearing of ﬂuids (e.g. lubricants) often yields rigid supramolecular structures, which can inversely affect ﬂuid properties such as viscosity (Apóstolo et al., 2019). Compression of ﬂuids ultimately leads to their solidiﬁcation. The solidiﬁcation of long chain alkanes at relatively low pressures imposes serious implications for their lubricating effects. In certain examples such as mixtures of squalene with polyα-oleﬁns, the solidiﬁcation at ca. 1 GPa is not associated with crystallization, but rather with the formation of a glassy state with reduced mobility. Other examples are known, such as with 1dodecanol, wherein moderate pressures induce crystallization of polymorphic modiﬁcations which favor sliding, and hence increase their lubricating ability: superlubricity (Reddyhoff et al., 2021). Finally, as in 1:1 mixtures of pentane:isopentane (Liu and Pulham, 2020), the pressure-induced solidiﬁcation of ﬂuids can be also selective and cause only a speciﬁc component to crystallize. As a result of this selective crystallization, the composition of the ﬂuid (in fact, a mesophase) changes. These examples serve to show just how interwoven the triboand mechano-sciences really are, and indeed the importance of understanding their connectivity. This vague borderline between “solid-state”and “ﬂuid-state” processes under mechanical action only exacerbates the problem of deﬁning the correct terminology to refer to a process involving mechanical action. Moreover, there is signiﬁcant likelihood that many reactions will need to be reclassiﬁed as our understanding of the mechanisms of mechanochemical reactions expands. To paraphrase N. Copernicus, we know what we know, but we do not Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857898 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry yet know what we do not know. This reality is of course impractical and indeed superﬂuous for most researchers who are more interested in the outcome of mechanical treatment rather than in the detailed mechanism of the transformation. What is important, however, is that the process can be reproduced by others based on the original, recorded description. This requires effective and accurate communication of the mechanochemical protocol used, with meaningful descriptions of all the parameters that are known to inﬂuence mechanochemical reactions. Hence, although we cannot know a priori how a seemingly “solid + solid”reaction will occur, we can know for sure how we treat the sample and analyze the outcome. It is also important to determine and record the appearance and state of the sample at the start and end of our treatment. It is this information rather than a name itself that must be reported and controlled as carefully as possible. Only in such a way can we hope to identify under what conditions solids react, and how to implement this technology most effectively. THE BREADTH OF MECHANOCHEMISTRY If we deﬁne a reaction as thermochemical, it is sufﬁcient to indicate the temperature at which it occurs. If the reaction is not isothermal, a protocol of modifying the temperature with time is required. For a photochemical reaction, energy, polarization, the intensity of light, the spatial characteristics of the irradiation (uniform, local, one-sided, etc.) must be deﬁned. In contrast, where irradiation is discontinuous, the duration and frequency of light pulses must be stated. For a mechanochemical transformation the type of mechanical action, experimental conditions, the composition and appearance of samples are much broader than for a thermal, or a photochemical reaction (Figure 4). In fact, the very questions as to “how we treat a sample”and “in what state the reactant and product phases exist”are not easily deﬁned. Increasingly, new features are being identiﬁed to be crucial for determining the reaction pathway of mechanochemical FIGURE 4 | Representation of the breadth of mechanical actions which are frequently used in mechanochemistry. (A) A broad classiﬁcation of the types of mechanical action encountered in mechanochemical reactions. (B) Examples of common devices under each category. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 6857899 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry FIGURE 8 | (A, B) Comparison of a simple vibratory ball milling co-crystal formation. (A) The original nomenclature (Michalchuk et al., 2017)and(B) the proposed nomenclature. (C–D) Comparison of nomenclature for the BUMS of Au nanoparticles by planetary ball milling with (C) original nomenclature and (D) recast using the proposed pictograms. Note the choice of planetary milling is only clear in the latter. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 68578916 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry FIGURE 9 | (A–D) Reaction schemes for the mechanochemical disulﬁde bond formation. (A, B) adapted from literature, Ref (Belenguer et al., 2019b). (C, D) The same reactions using the proposed general symbols for mechanochemistry. (E) Reaction schemes for ionic exchange in inorganic salts in a vibrational mill (Lampronti et al., 2021). FIGURE 10 | Using the proposed nomenclature to describe mechanochemical transformations with different types of additives. (A) An example of liquid assisted grinding formation of Ce based framework (Wilke et al., 2018)(B) An example of ionic liquid assisted grinding for the formation of caffeine + glutaric acid cocrystals (Mukherjee et al., 2018)(C) An example of polymer assisted grinding for the formation of phenazine + mesaconic acid cocrystals (Hasa et al., 2015). Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 68578917 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry method and show the immediate recognition of these three concepts, schematically, Figure 10. Without the need to deeply consider the text, or indeed understand the physical nature of the additive, the reader is immediately aware how these three vibratory ball milling reactions differ. Variation in temperature is also readily visible through use of the proposed nomenclature. This can be exempliﬁed by the effects of cryo-temperatures on milling of ϵ-chlorpropamide, Figure 11 (Drebushchak et al., 2011). As previously discussed, the proposed nomenclature not only provides a thorough understanding of the mechanochemical conditions, but allows the reader immediate recognition of the role of temperature on this polymorphic transformation. The reaction temperature is not always controlled externally, but can also be generated internally from the sample itself as in the case of self-sustaining reactions (Takacs, 2002). In the self-sustaining mechanosynthesis of TiB 2 for example (Park et al., 1994;Radev and Klisurski, 1994;Takacs, 2002), the amount of time required to ignite the powder depended critically on the type of mechanical action used: 80 min by planetary ball milling, but over 100 h by vibratory ball milling, Figure 11C.This unique origin of reaction temperature is clearly marked using the pictographic approach. FIGURE 11 | (A) Using the proposed nomenclature to describe mechanochemical transformations that differ according to temperature. (B) An illustration of the change in molecular conformations on cooling, which is reversible without a mechanical treatment, but is preserved (interlocked) after the molecular layers have been shifted on cryogrinding. (C) The self-sustaining mechanosynthesis of TiB 2 under planetary and vibratory ball milling conditions. FIGURE 12 | Using the proposed nomenclature to describe mechanochemical transformations that differ according to mechanoreactor. (A) Shearing action leads to dissociation of the 2:1 piroxiam:succinic acid cocrystal phase into its coformers, whereas impact action on powder of the coformers leads to formation of the cocrystal. (B) Difference in mechanical reactivity of ammonium perchlorate to impact and friction. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 68578918 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry The need to specify explicitly the type of mechanical action within mechanochemical protocol is exempliﬁed by the cocrystal formation of piroxicam and succinic acid, Figure 12A (Tumanov et al., 2014). The use of the triple milling ball symbol of Figure 12A would be wholly insufﬁcient in such cases. The differing effects of friction and impact are instead captured explicitly within the proposed system, and a reaction scheme based upon this nomenclature allows immediate recognition of this important mechanochemical phenomenon. The difference in mechanochemical reactivity of energetic materials is also exemplary of the need to make clear the type of mechanical action being applied. Ammonium perchlorate, for example, initiates ca. 5J mechanical impact, but remains insensitive (i.e. up to 360 N) when exposed to friction, Figure 12B (Roberts and Royle, 1991). The material of the milling jar and milling bodies can be important for the kinetics, the product composition (Germann et al., 2020) and apparent stability (Kulla et al., 2019), and the very possibility of a transformation on ball-milling. They can act as catalysts of the transformations of the powders which are hit by the balls. Such examples of mechanocatalysis have been documented both for inorganic (Grätz et al., 2020;Li et al., 1999;Chen et al., 2000) and organic (Pickhardtetal.,2020;Haley et al., 2016;Sawama et al., 2018;Cook et al., 2013;Chen et al., 2015;Vogt et al., 2021;ArdilaFierro and Hernández, 2021) compounds. For example, the oxidative coupling of tetrahydroisoquinolines with nitromethane by vibratory ball milling with solid additive DDQ was facilitated by using Cu milling balls as catalyst rather than adding another catalytic material to the reaction mixture, Figure 13A (Su et al., 2011). Similarly, the ball milling material can play an important role in thetreatmentofinorganicphases,asinthecaseofthepolymorphic transformation of Bixbyite (Y 2 O 3 ) under planetary ball milling conditions. When milled using steel balls, a monoclinic phase is formed, whereas the Fluorite structure is obtained when ZrO 2 milling bodies are used, Figure 13B (Begin-Colin et al., 1995). Also the mechanochemical formation of carbon allotropes (fullerenes, carbon nanotubes, carbon onion structures, etc.), which is known to be catalyzed by iron and iron alloys (Zhang et al., 2011;Zhang et al., 2012;Zhang et al., 2014;Velasquez et al., 2016), can be sensitive to the choice of mill (Güler and Evin, 2015), or the substitution of steel milling balls and jar for another material (Surov, 2004). Interestingly, the substitution of steel for ZrO 2 seemed to have no signiﬁcant effect on the mechanochemical synthesis of graphene oxide by neat ball milling of pristine graphite (Mahmoud et al., 2018). In all cases involving organic and inorganic reagents, these small details regarding the milling conditions can be again readily observed using the pictographic nomenclature. Mechanochemical techniques have now been applied to an enormous array of materials syntheses, well beyond what can be covered here. For example, mechanochemical covalent chemical reactions have been reported (Andersen and Mack, 2018b;Bolm and Hernández, 2019;Tan and García, 2019), spanning classical condensation (Haferkamp et al., 2019)orcyclization(Andersen and Mack, 2017) reactions through to metal-catalyzed (Pickhardt et al., 2020) or piezocatalyzed reactions (Kubota et al., 2019). Regarding inorganic materials, syntheses and modiﬁcations have been described forhydratedoxides(MacKenzie et al., 1999;Mehrotra et al., 2016; Alex et al., 2020), mixed oxides and ceramics (Balaz et al., 1994; Mackenzie et al., 2000;Temuujin et al., 2000;MacKenzie and Barneveld, 2006), silicates (Temuujin et al., 1998a;Temuujin et al., 1998b;MacKenzie et al., 2007), and high entropy alloys (Kumar et al., 2017;Balcerzak et al., 2019;Kamalakannan et al., 2019), amongst many others. We note that the pictographic representation is equally applicable to any reactions that occur as a result of mechanical action from across all aspects of chemical reactivity, Figure 14.Comparison of the pictographic representations for mechanochemical transformations of very different chemical species reveals immediately the diverse conditions required. It is readily apparent that inorganic compounds tend to be prepared using long duration planetary ball milling conditions under controlled atmospheres, whereas soft materials are prepared by gentler mechanical FIGURE 13 | Visualizing the effects of milling ball material on mechanochemical reactions. (A) Using the Cu milling balls as catalyst for an oxidative coupling reaction. (B) The formation of two unique polymorphic forms of Y 2 O 3 depending on the material of milling balls during planetary ball milling. Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 68578919 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry conditions and often beneﬁt from the addition of liquid additives. Hence, not only does this pictographic representation allows rapid identiﬁcation of experimental conditions being reported, but also offers a facile approach to identifying trends in experimental conditions successfully applied across the chemical and materials sciences. We therefore expect this clear and concise approach for representing mechanochemical transformations to facilitate new generalisations of mechanochemistry toward targeted and rapid design of new materials and molecular syntheses. SUMMARY The mechanochemical community has expanded signiﬁcantly in recent years. Throughout most of the 20th century, mechanochemistry was the focus of a smaller and relatively homogeneous scientiﬁc community of predominantly solid-state scientists. Now, the community of mechanochemists has ﬂourished, incorporating experts from a wide range of scientiﬁc backgrounds. To date, membership in the mechanochemistry ﬁeld includes researchers from all branches of chemistry, physics, materials sciences, pharmaceutical sciences, biological sciences, and engineering. Many of the newcomers are themselves trained experts in solution or gas phase reactivity, thereby bringing with them many unique viewpoints on phenomena of chemical reactivity. The diversiﬁcation of the mechanochemical community has triggered new, challenging and fundamentally important scientiﬁc questions and has led to mechanochemistry achieving more global impact than ever before. Mechanochemistry today is understood to include much more than what is strictly deﬁned by IUPAC as a “chemical reaction that is induced by the direct absorption of mechanical energy”(MechanoChemical Reaction, 2009). The term mechanochemistry has grown to include any transformation that is observed during or after any type of mechanical treatment, regardless of the exact role of the mechanical action. Moreover, the term mechanochemistry is applied equally to transformations which occur upon stretching of single molecules, through to transformations within and between solids (including those which involve ﬂuid intermediate states). Any transformation that is somehow facilitated by mechanical energy, or reactions that result from thermalor photochemically induced stress andstraininasolid[thechemomechanochemical effect (Boldyrev, 2018)] seem now to be denoted as “mechanochemical”.Thermalor photochemical transformations in solids which have been FIGURE 14 | Pictographic representation of literature mechanochemical reactions for a diverse selection of chemical systems, including planetary milling of alloys (Wang and Qin, 2003;Enayati et al., 2004), metal oxides (Stößer et al., 2013), and nitrides (Nash et al., 2016) (e.g. hexagonal boron nitride h-BN converting to defect laden hexagonal boron nitride dh-BN), vibratory ball milling of metal organic frameworks (Batzdorf et al., 2015), and Resonant Acoustic Mixing cocrystal synthesis (Michalchuk et al., 2018a). Frontiers in Chemistry | www.frontiersin.org May 2021 | Volume 9 | Article 68578920 Michalchuk et al. Tribochemistry, Mechanical Alloying, Mechanochemistry mechanically pre-treated are similarly denoted as being “mechanically activated”, and hence also fall within the current paradigm of mechanochemistry (Boldyrev, 2018;Galwey, 2020; Shields et al., 2020). With this growing diversity of the community and the phenomena being explored comes a confusion of scientiﬁc languages on the scale of the Tower of Babel. Terminologies and jargon used by experts from one discipline are often misunderstood by experts from a different background. Similarly, much of the over a century’s worth of research in mechanochemistry that is written in the scientiﬁc language of 20th century mechanochemical pioneers remains largely incomprehensible to many who enter the ﬁeld. Moreover, many of these original works have been not digitalized, and are thus not easily accessible until recently. Advances in modern digital technology have made these precious papers and their translations available via online platforms, thereby allowing the global community to stand on the shoulders of the ancestral mechanochemical giants. Despite these digital advances, there is still a large miscommunication between the established and emerging mechanochemistry communities. Many well-documented phenomena are unfortunately being regularly re-discovered, with many new terminologies being coined to describe them. Although in principle, science does not care how it is called, this has the knock-on effect of hindering how the scientiﬁc community can discuss, communicate, interpret, search the literature, and hence progress its collective understanding of the ﬁeld. For example, many who accomplish an organic synthesis in a mechanical device do not realize that they in fact deal not only with a chemical transformation, but with a plethora of tribochemical phenomena. By considering only one aspect of the whole, one risks to miss the elegance that nature has laid before us. For an elephant investigated in parts by blind men may be easily mistaken for a rope, a leaf, or a wall. In this same way, the strict isolation of triboand mechanochemistry exists only in the minds of humans. The growing interest in the mechanochemistry of organic compounds has revealed many new parameters which must be controlled to successfully achieve the reproducible mechanosynthesis of molecules and materials. These new parameters are of course in addition to those that were traditionally considered in tribochemistry and inorganic mechanochemistry. Parameters which are presently known to inﬂuence mechanochemical transformations include: the starting polymorph; the size and shape of particles; the type of mechanical action; the atmosphere under which the reaction has occurred; the presence and quantities of additives (solids, liquids, gases, polymers) even if they may not obviously participate in the reaction; the presence of catalysts, including those present as the materials of the milling bodies or reactors. Moreover, the speciﬁc parameters associated with the type of mechanical actions (e.g. revolutions per minute in twin screw extrusion or planetary ball milling, or frequency in ball milling, or the number, size and mass of the balls) are certainly important in deﬁning the reaction. Yet, many such parameters are often overlooked, remain unreported, or are difﬁcult to identify in literature reports. Although efforts at unraveling the mechanistic aspects and driving forces of mechanochemical research are without doubt of central importance, effective communication of protocol and processes do not require such an understanding. It might not be possible to classify unambiguously a reaction as “mechanochemical”, “tribochemical”or “mechanically facilitated thermochemical” without this mechanistic understanding, but we can still impose clarity of communication when reporting our results. Following the successful example of the Crystallographic Information File (CIF), we demonstrate how adopting a standard format for reporting experimental conditions can help ensure that important parameters are both monitored and controlled. Such agreements on the type of information that needs to be presented and how it should be presented so that it is clear to all on ﬁrst glance, are crucial to unify the community and drive fundamental developments in the ﬁeld of mechanochemistry. We believe this opinion piece will spark timely and productive discussion across the ever growing and diversifying mechanochemistry community. All symbols displayed in this text are available free of charge for those who wish to use them. Additional pictograms and information will certainly become important as new features of control over mechanochemical reactions emerge, and we as a community must be ready to adapt our nomenclature and standard of practice to accommodate this. As mechanochemistry becomes an increasingly integral part of efforts to develop environmentally benign strategies for chemical and industrial processes, it is timely to make efforts to unite the dynamic community behind common concepts and deﬁnitions in scientiﬁc language, and terminology. Only in doing so can we hope to avoid the potential downfall of the “Tower of Babel”and construct a coherent and robust “Tower of Mechanochemistry”. It is only by uniting the community that we can collectively obtain knowledge as, in the words of N. Copernicus, “to know that we know what we know, and that we do not know what we do not know, that is true knowledge.” AUTHOR’S NOTE A library of the pictograms used in this study are available free of charge from https://opus4.kobv.de/opus4-bam/frontdoor/index/ index/docId/52329 AUTHOR CONTRIBUTIONS All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication. ACKNOWLEDGMENTS As the founding president of the IMA (VB) and as members of the IMA and COST Action CA18112, the authors acknowledge support of the ongoing efforts of both organisations to develop a global community of mechanochemists. The authors are grateful to many signiﬁcant and seminal contributions to the ﬁeld of mechanochemistry by many remarkable scientists, upon whose shoulders the current ﬁeld stands. 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