Structural, Vibrational, and Magnetic Characterization of Orthoferrite LaFeO3 Ceramic Prepared by Reaction Flash Sintering

Citation: Manchón-Gordón, A.F.; Sánchez-Jiménez, P.E.; Blázquez, J.S.; Perejón, A.; Pérez-Maqueda, L.A. Structural, Vibrational, and Magnetic Characterization of Orthoferrite LaFeO3Ceramic Prepared by Reaction Flash Sintering. Materials 2023,16, 1019. https://doi.org/ 10.3390/ma16031019 Academic Editors: Rujie He, Wenjie Li, Qingbo Wen and Fei Li Received: 26 December 2022 Revised: 13 January 2023 Accepted: 19 January 2023 Published: 22 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Structural, Vibrational, and Magnetic Characterization of Orthoferrite LaFeO3Ceramic Prepared by Reaction Flash Sintering Alejandro F. Manchón-Gordón1,* , Pedro E. Sánchez-Jiménez 1,2,* , Javier S. Blázquez 3, Antonio Perejón1,2 and Luis A. Pérez-Maqueda 1 1Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, C. Américo Vespucio 49, 41092 Sevilla, Spain 2Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain 3Dpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain *Correspondence: [email protected] (A.F.M.-G.); pedr[email protected] (P.E.S.-J.) Abstract: LaFeO 3 perovskite ceramics have been prepared via reaction flash technique using Fe 2 O 3 and La 2 O 3 as precursors. The obtained pellets have been investigated using several techniques. The formation of LaFeO 3 has been clearly confirmed by X-ray diffraction. The scanning electron microscopy micrographs have shown the microporous character of the obtained pellets due to the low temperature and dwell time used in the synthesis process (10 min at 1173 K). The orthorhombic-rhombohedral phase transition has been observed at approximately 1273 K in differential thermal analysis measurements, which also allows us to determine the Néel temperature at 742 K. The fitted Mössbauer spectra exposed the presence of a single sextet ascribed to the Fe +3 ions in the tetrahedral site. Finally, magnetic measurements at room temperature indicate the antiferromagnetic character of the sample. Keywords: reactive flash sintering; lanthanum ferrite; perovskite structure; Mössbauer spectroscopy; magnetic properties 1. Introduction Oxides with the AFeO 3 (A = rare-earth) perovskite structure (the so-called orthoferrites) have been widely studied for their interesting properties and potential applications, such as catalysts [ 1 ] and gas sensors [ 2 ]. Among them, lanthanum orthoferrite (LaFeO 3 ) has been extensively analyzed because it exhibits ferroelectric and ferromagnetic properties [ 3 ], similar to those found in BiFeO 3 -based compounds [ 4 ]. In particular, LaFeO 3 and LaFeO 3 -based compounds are p-type semiconductors with outstanding physical and chemical properties [ 5 ], which have been employed as sensors [ 6 ], electrode materials in solid fuel cells [7] and photocatalysts [8]. LaFeO 3 crystallizes in an orthorhombically distorted perovskite structure with the Pnma space group and is a canted antiferromagnetic material, because of superexchange interaction between neighboring Fe 3+ ions. This compound exhibits a Néel temperature at TN∼ 740 K [ 9 ], which is the highest in the orthoferrite family. The interesting magnetic properties of this family of compounds have led them to be proposed, very recently, as promising candidates in the field of magnetic refrigeration [ 10 ] and as potential fillers for polymer nanocomposite magnets [11]. The performance of LaFeO 3 commonly is subordinated to its structure, particle shape, and size. Consequently, the synthesis and processing methods employed in its preparation have a great influence on the resulting properties [ 12 – 14 ]. In this sense, diverse methods for the preparation of perovskite-type LaFeO 3 have been described, based on the targeted applications [ 15 ]. This material is generally prepared by solid-state reaction technique [ 16 ], which Materials 2023,16, 1019. https://doi.org/10.3390/ma16031019 https://www.mdpi.com/journal/materials Materials 2023,16, 1019 2 of 12 commonly leads to the presence of secondary phases in ferrites [ 17 ]. Moreover, it often requires various heating and grinding steps to guarantee the homogeneous mixing of the oxide precursors employed [ 18 – 20 ], with posterior calcination at temperatures higher than 1773 K for 6 h [ 3 ]. Alternative synthesis techniques, such as combustion [ 21 ], mechanochemistry [22] and soft-chemistry [23] have been proposed to circumvent those limitations and facilitate the preparation of lanthanum ferrites free of secondary phases and nanostructured. Yet, high sintering temperatures are still required to prepare fully dense LaFeO 3 specimens due to poor sinterability [ 13 ]. Spark Plasma Sintering (SPS) can be used to reduce sintering temperature down to 923 K but at the expense of modification in the electrical properties due to the highly reducing environment in the SPS chamber [ 24 ]. A recently proposed current-assisted sintering technique, Flash Sintering (FS), has demonstrated the capability to significantly reduce the sintering temperature of ceramic materials [ 25 ]. In this technique, first proposed in 2010, heat and a modest electric field are applied simultaneously to the sample, through which a certain current density flow. At a certain temperature, which depends on several factors such as the applied electric field and the intrinsic conductivity of the material, a dramatic decrease in the electrical resistance of the specimens occurs, generally accompanied by rapid densification [ 26 ]. Since its proposal, FS has grabbed the interest of the scientific community, which has devoted an extraordinary effort to the development of the technique. For example, this technique has been extended to 3D complex-shaped specimens that go beyond the typical dog bones or disks, using a multiphase power supply and multiple electrodes [ 27 ]. Furthermore, Gil-González et al. demonstrated the feasibility of employing FS to produce dense and single-phase complex oxides in a single step from their precursors [ 28 ]. This new methodology, which has been denominated Reactive Flash Sintering (RFS), has been recently employed in the preparation of an extensive range of materials, such as ferrites [ 29 , 30 ], ferroelectric materials [ 31 , 32 ], ceramic composites [ 33 , 34 ], high-entropy oxides [ 35 – 37 ] and solid electrolytes [ 38 , 39 ]. FS and RFS techniques have been also employed for the preparation of LaFeO 3 -based compounds. For example, it has been achieved the sintering of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 at temperatures lower than 373 K [ 40 ] and the co-sintering of a lanthanum strontium cobalt ferrite/Gd-doped ceria bilayer structure by FS [ 41 ]. It has been also reported the preparation of lanthanum strontium cobalt ferrite via reaction flash light sintering, in which the material is exposed to a xenon arc lamp [ 42 ]. However, to the best of our knowledge, the preparation of phase-pure LaFeO 3 perovskite by RFS has not been reported. The purpose of this work is to further demonstrate the capability of RFS for the preparation of multiferroic and magnetic materials which are hard to prepare by conventional techniques. Thus, we show that phase-pure LaFeO 3 pellets can be indeed prepared by this technique. The crystal structure, microstructure, thermal, vibrational, and magnetic properties of the obtained pellets were characterized. Mössbauer spectroscopy has been used to obtain selective information about the iron ions, such as the oxidation state, coordination, and the existence of impurity phases in the prepared LaFeO3pellets. 2. Materials and Methods LaFeO 3 was synthesized from commercially available α -Fe 2 O 3 (Sigma Aldrich, Darmstadt, Germany; <5 µ m, ≥ 99% purity) and La 2 O 3 (Sigma Aldrich, Darmstadt, Germany; ≥ 99.9% purity) powders. The oxides were dried in a warming oven for 0.5 h to remove any moisture and mixed by ball milling in stoichiometric proportions. To prepare the pellets, approximately 0.8 g of the precursor powder was mixed with a polyvinyl alcohol binder solution and uniaxially pressed into dog-bone shapes under a pressure of 500 MPa for 3 min . Subsequently, the obtained dog bones were pre-heated at 723 K for 0.5 h in the air to remove the binder prior to the RFS experiment. Electrodes consisted of two platinum wires attached to the handles of the dog bone and connected to a 1500 W DC power supply (EA-PSI 9760-06 DT). The RFS experiment was carried out in a tubular furnace keeping constant the temperature while the intensity current limit was progressively increased at 10 mA s −1 . Once the selected Materials 2023,16, 1019 3 of 12 intensity current limit was reached, the flash state was maintained for 10 min. After this time, the furnace and power supply were shut down. The preparation of LaFeO 3 was followed, at room temperature, by X-ray diffraction technique using a Rigaku MiniFlex diffractometer with Cu-K α radiation, λ = 1.5406 ˚ A . Phase transition temperatures were determined from differential scanning calorimetry (DSC) measurements, using a simultaneous TG/DSC (Q650 SDT, TA Instruments) under a nitrogen flow and a heating rate of 10 K/min. Scanning electron microscopy (SEM) was employed to study the microstructure of the obtained pellets using an FEI Teneo microscope. To do that, the surface morphology of the fracture of the samples was directly observed after a block was cut from the original pellet. 57 Fe Mössbauer spectroscopy was carried out at room temperature using a 57 Co(Rh) source in transmission geometry. Samples were measured as powders by mechanically grinding the pellets. NORMOS program [ 43 ] has been employed to determine hyperfine parameters. As it is conventionally done, isomer shift is given relative to an α -Fe foil. Raman spectroscopy was measured on the fracture surfaces of the samples using a dispersive microscope Raman Horiva Jobin Yvon LabRam HR800 operating at 532.1 nm. At least ten spectra were acquired from each sample. Magnetic properties were characterized in a Quantum Design Physical Properties Measurement System, PPMS, using the vibrating sample magnetometer standard option. Hysteresis loops at room temperature were carried out using a maximum applied magnetic field of 5 T. 3. Results and Discussion The preparation of the desired product by reactive flash sintering requires meticulous control of the experimental conditions [ 29 ] due to the thermal management during the flash event, which results in sample heterogeneities such as abnormal grain growth, preferential current paths, and local melting [ 44 ]. To avoid these problems, typical of conventional flash experiments in which the electric field is kept constant while the temperature is linearly increased, the use of current density ramps has been proposed [ 45 ]. This variation of the technique allows increasing the current density circulating through the material linearly with time while the applied voltage is automatically adjusted. Thus, it enables to pre-define the onset temperature, i.e., the temperature at which the flash event takes place. In this work, we have employed current density ramps to explore if it is viable to prepare single-phase LaFeO 3 orthorferrite via reaction flash sintering. Figure 1depicts the variation in typical electrical parameters involved in a flash experiment with time during the RFS process when a current density ramp is employed. A sharp increase in the electric field at the beginning of the process is clearly observed, which is necessary to follow the imposed current density ramp. Then, the electric field decreases to a stable value when the preset current density value is reached. Similar behavior can be observed in the case of the power density dissipated, P=JE, where Jis the current density and Ethe electric field. Several experiments have been performed to determine the optimum flash conditions, using different combinations of onset temperature and current density limits. Obtained results have been depicted in Figure 2. As can be observed, only under very specific conditions, 1173 K and 50 mA mm −2 , phase-pure samples can be prepared. In any other case, the explored experimental conditions result in incomplete reactions, as will be discussed below, or in samples exhibiting current localization or hot spots. The latter involves the formation of preferential paths for the current flow that results in highly heterogeneous specimens [29]. The crystal structure and composition of the prepared LaFeO 3 pellets were studied by X-ray diffraction at room temperature. The XRD profiles of specimens flashed at 1173 K with different current density limits are presented in Figure 3. As can be observed, if the maximum current density is set to 38 mA mm −2 , traces of the precursors (Fe 2 O 3 and La 2 O 3 ) are detected alongside the perovskite structure. This fact indicates that insufficient energy has been supplied to produce the desired LaFeO 3 phase. By increasing the current density, Materials 2023,16, 1019 4 of 12 the amount of precursors after the RFS process decreases, and by further increasing the maximum current density to 50 mA mm −2 , no traces of the precursors were detected, and phase-pure LaFeO3were obtained. Materials 2022, 15, x FOR PEER REVIEW 4 of 13 0 2 4 6 8 10 12 0 10 20 30 40 50 60 70 E (V cm-1) time (min) 0 10 20 30 40 50 60 J (mA mm-2) 0 50 100 150 200 P (mW mm-3) Figure 1. Evolution of reaction flash parameters, electric field, power density dissipation, and current density limit, as a function of time during the reactive flash process. 950 1000 1050 1100 1150 1200 1250 1300 35 40 45 50 55 Incomplete Localized Optimized J (mA mm-2) T (K) Figure 2. Experimental conditions tested in the reactive flash process of La2O3 and Fe2O3, in terms of onset temperature and current density limits (flash event holding 10 min). The crystal structure and composition of the prepared LaFeO3 pellets were studied by X-ray diffraction at room temperature. The XRD profiles of specimens flashed at 1173 K with different current density limits are presented in Figure 3. As can be observed, if the maximum current density is set to 38 mA mm−2, traces of the precursors (Fe2O3 and La2O3) are detected alongside the perovskite structure. This fact indicates that insufficient energy has been supplied to produce the desired LaFeO3 phase. By increasing the current Figure 1. Evolution of reaction flash parameters, electric field, power density dissipation, and current density limit, as a function of time during the reactive flash process. Materials 2022, 15, x FOR PEER REVIEW 4 of 13 0 2 4 6 8 10 12 0 10 20 30 40 50 60 70 E (V cm-1) time (min) 0 10 20 30 40 50 60 J (mA mm-2) 0 50 100 150 200 P (mW mm-3) Figure 1. Evolution of reaction flash parameters, electric field, power density dissipation, and current density limit, as a function of time during the reactive flash process. 950 1000 1050 1100 1150 1200 1250 1300 35 40 45 50 55 Incomplete Localized Optimized J (mA mm-2) T (K) Figure 2. Experimental conditions tested in the reactive flash process of La2O3 and Fe2O3, in terms of onset temperature and current density limits (flash event holding 10 min). The crystal structure and composition of the prepared LaFeO3 pellets were studied by X-ray diffraction at room temperature. The XRD profiles of specimens flashed at 1173 K with different current density limits are presented in Figure 3. As can be observed, if the maximum current density is set to 38 mA mm−2, traces of the precursors (Fe2O3 and La2O3) are detected alongside the perovskite structure. This fact indicates that insufficient energy has been supplied to produce the desired LaFeO3 phase. By increasing the current Figure 2. Experimental conditions tested in the reactive flash process of La 2 O 3 and Fe 2 O 3 , in terms of onset temperature and current density limits (flash event holding 10 min). Materials 2023,16, 1019 5 of 12 Materials 2022, 15, x FOR PEER REVIEW 5 of 13 density, the amount of precursors after the RFS process decreases, and by further increasing the maximum current density to 50 mA mm−2, no traces of the precursors were detected, and phase-pure LaFeO3 were obtained. 20 30 40 50 60 70 80 Intensity (a.u.) 2q (degrees) GOF=1.44 Space Group: Pnma (b) Figure 3. (a) XRD patterns of the RFS-ed samples under different current density limits at 1173 K. The indexed peaks correspond to the main diffraction peaks of the LaFeO3 compound (space group Pnma). The traces of the precursors have been also indicated. (b) Le Bail refinement of the XRD pattern corresponding to the phase-pure specimen. Le Bail refinement of the XRD pattern corresponding to the phase-pure specimen has been depicted in Figure 3b. All peaks could only be indexed by the Pnma space group. The obtained lattice parameters were a = 5.5603(2) Å, b = 7.8447(8) Å, c = 5.5438(4) Å and the crystal size was 119(2) nm. The obtained values of the crystallographic parameters are similar to those obtained for high-quality LaFeO3 samples[46]. 20 30 40 50 60 70 80 (412) (204) (143) (242) (331) (240) (311) (141) (230) (131) (202) (220) (112) (210) (121) (111) Intensity (a.u.) 2q (degrees) 38 mA mm-2 44 mA mm-2 50 mA mm-2 La2O3 Fe2O3 (101) (a) Figure 3. ( a ) XRD patterns of the RFS-ed samples under different current density limits at 1173 K. The indexed peaks correspond to the main diffraction peaks of the LaFeO 3 compound (space group Pnma). The traces of the precursors have been also indicated. ( b ) Le Bail refinement of the XRD pattern corresponding to the phase-pure specimen. Le Bail refinement of the XRD pattern corresponding to the phase-pure specimen has been depicted in Figure 3b. All peaks could only be indexed by the Pnma space group. The obtained lattice parameters were a= 5.5603(2) ˚ A ,b= 7.8447(8) ˚ A ,c= 5.5438(4) ˚ A and the crystal size was 119(2) nm. The obtained values of the crystallographic parameters are similar to those obtained for high-quality LaFeO3samples [46]. Materials 2023,16, 1019 6 of 12 The phase transitions of the prepared samples were studied by differential scanning calorimetry. Figure 4shows the DTA scans at 10 K min −1 of the phase-pure sample both on heating and cooling. The weak endothermic peak around 742 K corresponds to the Néel temperature, TN , which indicates the antiferromagnetic-paramagnetic transition. The second endothermic peak, much more intense, that appears around 1262 K is associated with the orthorhombic-rhombohedral phase transition. No more peaks, which could have revealed the existence of impurities, have been detected. In fact, both TN and the temperature of the phase transition is in agreement with those reported in the literature for high-quality LaFeO3samples [13,47]. Materials 2022, 15, x FOR PEER REVIEW 6 of 13 The phase transitions of the prepared samples were studied by differential scanning calorimetry. Figure 4 shows the DTA scans at 10 K min−1 of the phase-pure sample both on heating and cooling. The weak endothermic peak around 742 K corresponds to the Néel temperature, 𝑇𝑁, which indicates the antiferromagnetic-paramagnetic transition. The second endothermic peak, much more intense, that appears around 1262 K is associated with the orthorhombic-rhombohedral phase transition. No more peaks, which could have revealed the existence of impurities, have been detected. In fact, both 𝑇𝑁 and the temperature of the phase transition is in agreement with those reported in the literature for high-quality LaFeO3 samples [13,47]. 600 800 1000 1200 1400 Heat flow T (K) 0.2 W/g exo up TN 1230 1260 1290 Heat flow T (K) 0.1 W/g Figure 4. DTA scans at 10 K min−1 for phase-pure LaFeO3 obtained by RFS. Inset shows a horizontally zoomed plot in a limited range of temperatures (black and red lines correspond to heating and cooling, respectively). Figure 5a depicts representative SEM images of the cross-section fracture of the phase-pure LaFeO3 specimen. The micrograph exhibits an inhomogeneous distribution of grains. Thus, the grains are of irregular shape, and the sizes are lower than 1 μm. In fact, the small grain size in RFS-ed samples is typical of ferrites produced by this technique due to the low sintering temperature and time needed, which limits grain growth [30,48,49]. The observed microstructure is consistent with a not totally sintered material since some micropores have been clearly detected. However, it is significant the relative density is around 80% (estimated from Archimedes method) considering the time and temperature employed in the experiments carried out in this work. In fact, it has been reported that obtaining relative densities around 90% in LaFeO3 requires long dwell times and high temperatures (1673 K and 10 h) [13], much higher than the furnace temperature of 1173 K required by RFS. This entails substantial energy saving [49]. In this sense, although the reaction of the specimen prepared at a furnace temperature of 1273 K was incomplete, the sample exhibits higher grain size and density (Figure 5c). On the contrary, lower temperatures induce lower grain size and density (Figure 5b). In Figure 5d, the effect of the current localization on the morphology of the specimen can be observed. Figure 4. DTA scans at 10 K min −1 for phase-pure LaFeO 3 obtained by RFS. Inset shows a horizontally zoomed plot in a limited range of temperatures (black and red lines correspond to heating and cooling, respectively). Figure 5a depicts representative SEM images of the cross-section fracture of the phasepure LaFeO 3 specimen. The micrograph exhibits an inhomogeneous distribution of grains. Thus, the grains are of irregular shape, and the sizes are lower than 1 µ m. In fact, the small grain size in RFS-ed samples is typical of ferrites produced by this technique due to the low sintering temperature and time needed, which limits grain growth [ 30 , 48 , 49 ]. The observed microstructure is consistent with a not totally sintered material since some micropores have been clearly detected. However, it is significant the relative density is around 80% (estimated from Archimedes method) considering the time and temperature employed in the experiments carried out in this work. In fact, it has been reported that obtaining relative densities around 90% in LaFeO 3 requires long dwell times and high temperatures (1673 K and 10 h) [ 13 ], much higher than the furnace temperature of 1173 K required by RFS. This entails substantial energy saving [ 49 ]. In this sense, although the reaction of the specimen prepared at a furnace temperature of 1273 K was incomplete, the sample exhibits higher grain size and density (Figure 5c). On the contrary, lower temperatures induce lower grain size and density (Figure 5b). In Figure 5d, the effect of the current localization on the morphology of the specimen can be observed. Materials 2023,16, 1019 7 of 12 Materials 2022, 15, x FOR PEER REVIEW 7 of 13 Figure 5. SEM micrographs of cross-section fractures of the specimens obtained using a maximum current density of 50 mA mm−2 at (a) 1173 K (optimized conditions), (b) 1073 K, (c) 1273 K. (d) Micrograph that shows the effect of current localization (56 mA mm−2 at 1173 K). The characteristic crystal structure of the orthoferrite compounds makes Raman spectroscopy an extremely useful technique to obtain relevant information on various dynamical processes involving phonons, charge carriers, and spins [50–52]. On the other hand, Raman spectroscopy is known to be a powerful technique to determine the structure distortion and oxygen motion of perovskite-type materials [53]. Although no Raman modes would be expected in the case of an ideal cubic ABO3 perovskite structure, the characteristic distortion of the orthorhombic structure induces the activation of Raman vibration modes, in which atoms occupy four non-equivalent positions and only B atoms are situated at the center of symmetry in the corner-sharing BO6 octahedral network [54,55]. Figure 6 shows the Raman spectrum of the phase-pure LaFeO3 sample taken at room temperature. The position of the observed modes has been collected in Table 1, which is in good agreement with those reported in the literature [53,56,57]. In LaFeO3 ceramics, Raman modes lower than 200 cm−1 are ascribed to the deformation motivated by displacement of La cations. The vibrational modes close to 300 cm−1 include several components and it is ascribed to the FeO6 octahedral tilt modes, while the mode around 450 cm−1 is associated with the oxygen octahedral bending vibrations. Raman mode centered around 520 cm−1 belongs to Ag symmetry and the mode observed at 630 cm−1 reflects oxygen stretching vibrations. Finally, the broad features found above 1000 cm−1 are attributed to second-order scattering [21,57]. Figure 5. SEM micrographs of cross-section fractures of the specimens obtained using a maximum current density of 50 mA mm −2 at ( a ) 1173 K (optimized conditions), ( b ) 1073 K, ( c ) 1273 K. (d) Micrograph that shows the effect of current localization (56 mA mm−2at 1173 K). The characteristic crystal structure of the orthoferrite compounds makes Raman spectroscopy an extremely useful technique to obtain relevant information on various dynamical processes involving phonons, charge carriers, and spins [ 50 – 52 ]. On the other hand, Raman spectroscopy is known to be a powerful technique to determine the structure distortion and oxygen motion of perovskite-type materials [ 53 ]. Although no Raman modes would be expected in the case of an ideal cubic ABO 3 perovskite structure, the characteristic distortion of the orthorhombic structure induces the activation of Raman vibration modes, in which atoms occupy four non-equivalent positions and only B atoms are situated at the center of symmetry in the corner-sharing BO6octahedral network [54,55]. Figure 6shows the Raman spectrum of the phase-pure LaFeO 3 sample taken at room temperature. The position of the observed modes has been collected in Table 1, which is in good agreement with those reported in the literature [ 53 , 56 , 57 ]. In LaFeO 3 ceramics, Raman modes lower than 200 cm −1 are ascribed to the deformation motivated by displacement of La cations. The vibrational modes close to 300 cm −1 include several components and it is ascribed to the FeO 6 octahedral tilt modes, while the mode around 450 cm −1 is associated with the oxygen octahedral bending vibrations. Raman mode centered around 520 cm −1 belongs to A g symmetry and the mode observed at 630 cm −1 reflects oxygen stretching vibrations. Finally, the broad features found above 1000 cm −1 are attributed to second-order scattering [21,57]. Materials 2023,16, 1019 8 of 12 Materials 2022, 15, x FOR PEER REVIEW 8 of 13 200 400 600 800 1000 1200 1400 1600 Intensity (a.u.) Raman Shift (cm-1) Second order scattering B1g Ag(3) B1g Ag(4) Ag(5) B1g(5) Figure 6. Raman spectrum of phase-pure LaFeO3 at room temperature. Table 1. Position of Raman modes and their assignments for the phase-pure LaFeO3 compound prepared by RFS. Raman Modes Raman Shift (cm−1) B1g(5) 166 Ag(5) 188 Ag(4) 274 301 334 B1g(3) 438 B3g(3) 486 Ag(3) 515 B1g 631 Second order scattering 1133 1305 Figure 7 depicts the Mössbauer spectrum of the phase-pure LaFeO3 specimen taken at room temperature. It can be observed that the spectrum exhibits a single sharp sextet, which has been fitted with a single sextet. The hyperfine parameters denote that iron in the LaFeO3 compound presents a valence state 3+, indicated by the obtained values of isomer shift, 𝛿 = 0.25(2) mm s−1. In fact, a valence state 2+ is indicated for values of 𝛿 > 0.9 mm s−1 [58]. On the other hand, the high magnetic hyperfine field obtained, 𝐵ℎ𝑓 = 52.2(3) T, indicates a strong Fe-Fe coupling, suggesting an antiferromagnetic behavior of the studied compound, as will be discussed below. Additionally, the small value of the quadrupole splitting, QS = −0.08(2) mm s−1, suggests the distortion of the octahedral arrangement of oxygens [56], in agreement with the Raman results. No other contributions have been detected by Mössbauer spectroscopy, indicating that the sample is phase-pure, in agreement with the XRD and DSC results presented above. The obtained values of hyperfine parameters are in agreement with those described in the literature [56,59]. Figure 8 depicts the hysteresis loop of the LaFeO3 phase-pure sample taken at room temperature under an applied magnetic field ranging between −5 and 5 T. As it was expected, the studied specimen exhibits an almost linear field dependence of magnetization, typical of antiferromagnetic materials, which indicates that the magnetization (or remnant magnetization) is almost zero, confirming the antiferromagnetic character of this sample Figure 6. Raman spectrum of phase-pure LaFeO3at room temperature. Table 1. Position of Raman modes and their assignments for the phase-pure LaFeO 3 compound prepared by RFS. Raman Modes Raman Shift (cm−1) B1g(5) 166 Ag(5) 188 Ag(4) 274 301 334 B1g(3) 438 B3g(3) 486 Ag(3) 515 B1g 631 Second order scattering 1133 1305 Figure 7depicts the Mössbauer spectrum of the phase-pure LaFeO 3 specimen taken at room temperature. It can be observed that the spectrum exhibits a single sharp sextet, which has been fitted with a single sextet. The hyperfine parameters denote that iron in the LaFeO 3 compound presents a valence state 3+, indicated by the obtained values of isomer shift, δ = 0.25(2) mm s −1 . In fact, a valence state 2+ is indicated for values of δ > 0.9 mm s −1 [ 58 ]. On the other hand, the high magnetic hyperfine field obtained, Bh f = 52.2(3) T, indicates a strong Fe-Fe coupling, suggesting an antiferromagnetic behavior of the studied compound, as will be discussed below. Additionally, the small value of the quadrupole splitting, QS = − 0.08(2) mm s −1 , suggests the distortion of the octahedral arrangement of oxygens [ 56 ], in agreement with the Raman results. No other contributions have been detected by Mössbauer spectroscopy, indicating that the sample is phase-pure, in agreement with the XRD and DSC results presented above. The obtained values of hyperfine parameters are in agreement with those described in the literature [56,59]. Materials 2023,16, 1019 9 of 12 Materials 2022, 15, x FOR PEER REVIEW 9 of 13 caused by strong Fe-Fe couplings [60]. However, some hysteresis can be observed, which could be related to the presence of vacancies, typical of samples prepared by flash sintering [61,62], which affect the Fe-Fe couplings. In fact, the absolute magnetization of the sample is less than 0.5 emu g−1 even at high applied fields. The observed unsaturation of the sample at the maximum applied field is due to the inconsistent direction of the moments, which are also signals of antiferromagnetism. -10 -5 0 5 10 Intensity (a.u.) velocity (mm/s) Figure 7. Experimental room temperature Mössbauer spectrum (symbols) and model fitting (lines) of the resulting LaFeO3 pellet obtained after the reaction flash sintering process. Figure 8. Magnetic hysteresis loop of the resulting LaFeO3 pellet obtained after the reaction flash sintering process. -60000 -40000 -20000 0 20000 40000 60000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 s (emu g-1) Magnetic Field (Oe) Figure 7. Experimental room temperature Mössbauer spectrum (symbols) and model fitting (lines) of the resulting LaFeO3pellet obtained after the reaction flash sintering process. Figure 8depicts the hysteresis loop of the LaFeO 3 phase-pure sample taken at room temperature under an applied magnetic field ranging between − 5 and 5 T. As it was expected, the studied specimen exhibits an almost linear field dependence of magnetization, typical of antiferromagnetic materials, which indicates that the magnetization (or remnant magnetization) is almost zero, confirming the antiferromagnetic character of this sample caused by strong Fe-Fe couplings [ 60 ]. However, some hysteresis can be observed, which could be related to the presence of vacancies, typical of samples prepared by flash sintering [ 61 , 62 ], which affect the Fe-Fe couplings. In fact, the absolute magnetization of the sample is less than 0.5 emu g −1 even at high applied fields. The observed unsaturation of the sample at the maximum applied field is due to the inconsistent direction of the moments, which are also signals of antiferromagnetism. Materials 2022, 15, x FOR PEER REVIEW 9 of 13 caused by strong Fe-Fe couplings [60]. However, some hysteresis can be observed, which could be related to the presence of vacancies, typical of samples prepared by flash sintering [61,62], which affect the Fe-Fe couplings. In fact, the absolute magnetization of the sample is less than 0.5 emu g−1 even at high applied fields. The observed unsaturation of the sample at the maximum applied field is due to the inconsistent direction of the moments, which are also signals of antiferromagnetism. -10 -5 0 5 10 Intensity (a.u.) velocity (mm/s) Figure 7. Experimental room temperature Mössbauer spectrum (symbols) and model fitting (lines) of the resulting LaFeO3 pellet obtained after the reaction flash sintering process. Figure 8. Magnetic hysteresis loop of the resulting LaFeO3 pellet obtained after the reaction flash sintering process. -60000 -40000 -20000 0 20000 40000 60000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 s (emu g-1) Magnetic Field (Oe) Figure 8. Magnetic hysteresis loop of the resulting LaFeO 3 pellet obtained after the reaction flash sintering process. 4. Conclusions Reaction flash sintering has been successfully employed to produce phase-pure orthorhombic LaFeO 3 compound in a single step in just 10 min holding time. The X-ray