Properties Design: Prediction and Experimental Validation of the Luminescence Properties of a New Eu(II)‐Based Phosphor

COMMUNICATION Properties Design: Prediction and Experimental Validation of the Luminescence Properties of a New Eu(II)-based Phosphor A. García-Fuente, F. Baur, F. Cimpoesu, A. Vega, T. Jüstel, W. Urland [a] Abstract: We present here our theoretical model that allows to predict, for the first time, the luminescence properties of a new phosphor (BaSnSi3O9:Eu2+) before the experiment is performed. The predicted emission wavelength, 488 nm with a 64 nm bandwidth, is confirmed by subsequent experimental work. The method consists in a multielectronic Hamiltonian parametrized from ab initio calculations. The luminescence properties of other similar compounds (BaHfSi3O9:Eu2+ and BaZrSi3O9:Eu2+), for which there is already experimental information, are also correctly reproduced. The development of new phosphors with novel luminescence properties is a distinct challenge of applied research.1-5 New phosphors are essential for the design of new lighting devices, but they have also become relevant in other fields such as medicine,6,7 horticulture,7,8 photovoltaics9 or quantum information technology.10 Certain lanthanide ions, as is the case of Eu2+, are well suited by the placement in visible spectra of emissions due to electric-dipole allowed [Xe]4fn[Xe]4fn-15d1 transitions. The particular host plays a key role in the fine tuning of absorption vs emission energies. The theoretical calculation of the luminescence properties of lanthanide ions has also attracted a lot of attention, both with semi-empirical11,12 and ab initio methods.13-22 However, all these works focus on reproducing and understanding the available experimental data. We present here the first-ever theoretical prediction of a brand-new phosphor based on ab initio calculations, that is also confirmed by experimental data afterwards. The developed model will thus also allow to understand quantitatively unusual luminescent behavior, like a significant red shift, in host compounds doped with Eu3+ which has been interpreted only qualitatively.23 In particular, we focus on Eu2+ doped host compounds BaMSi3O9 (M=Hf, Zr, Sn). The given structure type is depicted in Figure 1. It presents one Ba2+ site that can be used for Eu2+ doping. In that case Eu2+ is surrounded by six SiO44groups, with the closest six oxygen atoms at the same distance, forming a face-compressed octahedron with D3 local symmetry. The phosphor BaHfSi3O9:Eu2+ has already been experimentally synthesized and tested, showing an intense blue-green emission centered at 475 nm.24 The BaZrSi3O9:Eu2+ material has the emission peak between 475 nm25 and 479 nm,26 according to different syntheses. This work started from the idea of predicting a new phosphor completely from first principles using ab initio calculations and well-tempered phenomenological Hamiltonians, verifying a posteriori the a priori predictions. Thus, we proposed the BaSnSi3O9:Eu2+ system, for which there was no preliminary experimental data. The deal is corroborated with the reproduction of the available experimental data for BaHfSi3O9:Eu2+ and BaZrSi3O9:Eu2+. Figure 1. Representation of the BaMSi3O9 (M=Hf, Zr, Sn) structure (left) and of the local structure around the Eu2+ activator (right) showing local D3 symmetry. Light blue, red, green, grey and purple spheres are used to represent Ba, O, Si, M and Eu atoms, respectively. The modelling is a two-step process. The first step consists in the structural optimization, by density functional theory (DFT) calculations, of a periodic supercell of BaMSi3O9 (M = Hf, Zr, Sn) doped with Eu2+ in a Ba2+ site. A non-routine essential aspect is the DFT emulation of both the ground electronic configuration 4f7 (GC) and the excited electronic configuration 4f65d (EC) of Eu2+. Table 1 shows the Eu-O coordination bond lengths in the different structures, as function of the established configuration. Note that the EC structures always present shorter bond distances than the GC structures, what seems apparently counterintuitive. However, this phenomena has already been reported both theoretically15,27 and experimentally28,29 for other lanthanide doped phosphors. The fact is that because, while the f-shell does not contribute to chemical bonding, the formally 5d virtuals are responsible for the stabilization of f-type complexes. Consequently, the f6d EC states are more bonded, with shorter Eu-O contacts. [a] A. García-Fuente Departamento de Física Universidad de Oviedo, E-22007 Oviedo, Spain. F. Baur Department of Chemical Engineering Münster University of Applied Sciences, D-48565 Steinfurt, Germany. F. Cimpoesu Institute of Physical Chemistry, 060021 Bucharest, Romania. A. Vega Departamento de Física Teórica, Atómica y Óptica Universidad de Valladolid, E-47011 Valladolid, Spain. T. Jüstel Department of Chemical Engineering Münster University of Applied Sciences, D-48565 Steinfurt, Germany. W. Urland Private Institute of Theoretical Chemical Physics, CH-6600 Muralto, Switzerland Supporting Information for this article is given via a link at the end of the document. COMMUNICATION Table 1. Relaxed distances between the Eu2+ activator and its closest oxygen atoms in the different BaMSi3O9:Eu2+ structures. System Eu-O (GC) Eu-O (EC) BaHfSi3O9:Eu2+ 2.556 Å 2.515 Å BaZrSi3O9:Eu2+ 2.547 Å 2.498 Å BaSnSi3O9:Eu2+ 2.548 Å 2.449 Å In a second step, we use the DFT results to parametrize a multielectronic Hamiltonian operator that describes the valence states of the Eu2+ ion embedded in the host. Our Hamiltonian can be decomposed as: 𝐻 = 𝐻0+𝐻𝐶𝐹 + 𝐻𝐸𝐸 + 𝐻𝑆𝑂 (1) where 𝐻0 and 𝐻𝐶𝐹 are one-electron terms, originating from intraatomic and crystal field, respectively, while 𝐻𝐸𝐸 and 𝐻𝑆𝑂 stand for electron-electron and spin-orbit interactions. We considered an extended configuration interaction (CI) constructed with Slater Determinants (SDs) made of pseudo-atomic wavefunctions of f and d types. The basis is built merging the full CI within the 4f7 and 4f65d configurations, amounting 3432 SDs and 30030 SDs, respectively. Although some luminescence properties of Eu2+-based phosphors can be obtained from simplified approaches,17,30 here we deal with the complete diagonalization of the 𝐻 matrix of dimension 33462, a computational challenge that only modern computers are able to achieve. As customary in Crystal Field (CF) Theory the inter-electron part is parameterized with the so-called Slater-Condon integrals.31,32 For the inter-shell effect, the 𝐹𝑘(4f,5d) elements (with k=0, 2, 4) are accounting for Coulomb interaction, while the 𝐺𝑘(4f,5d) (with k=1, 3, 5) are yielding the exchange part. Within the 4f shell, both effects are termed with the 𝐹𝑘(4f,4f) integrals (with k= 2, 4, 6). We calculate 𝐹𝑘 and 𝐺𝑘 by numerical integration of the radial 4f and 5d wavefunctions of the free Eu2+ ion as obtained from DFT calculations, with a correction introduced to reproduce the available experimental data.19 The results are shown in the second column of Table 2. HSO can be written in terms of a spin-orbit coupling parameter 𝜉(𝑛𝑙) for each electronic shell, obtained from the orbital splitting from DFT calculations including spin-orbit interaction19 (see Table 2). Due to the delocalization of the 5d states when the Eu2+ ion is embedded in the BaMSi3O9 host lattice, the 𝐹𝑘(4f,5d),𝐺𝑘(4f,5d) and 𝜉(5d) are reduced respect to their values for the free ion (that is, the nephelauxetic effect). We calculate a reduction factor to these parameters  from the amount of delocalization of the 5d states over the surrounding O2ions in the host. We find =0.91 for all our systems. In turn, the f-only parameters of the embedded ion are practically the same of the free one. Results are given in the third column of Table 2. The ∆(fd) parameter cumulates 𝐻0 and 𝐻𝐸𝐸 terms forming the traces of the Hamiltonain blocks based on 4f7 versus 4f65d manifolds. Since ∆(fd) is not influenced by CF effects, it will be fixed to the same value for all the different BaMSi3O9:Eu2+ compounds, based on the value fit from the 475 nm experimental emission wavelength in the BaHfSi3O9:Eu2+ case.24 Table 2. Slater-Condon parameters of the free Eu2+ ion (second column) and of the Eu2+ embedded in the BaMSi3O9 host lattice. Parameter free Eu2+ ion (cm-1) BaMSi3O9:Eu2+ (cm-1) 𝐹2(4f,4f) 385.1 385.1 𝐹4(4f, 4f) 49.8 49.8 𝐹6(4f,4f) 5.3 5.3 𝐺1(4f,5d) 256.2 233.2 𝐹2(4f,5d) 170.4 155.0 𝐺3(4f,5d) 22.5 20.5 𝐹4(4f, 5d) 12.8 11.7 𝐺5(4f,5d) 3.5 3.2 𝜉(4f) 1363 1363 𝜉(5d) 806 733 ∆(fd) 5576 -3345 The essential 𝐻𝐿𝐹 part in the multielectronic Hamiltonian was tackled taking directly from the  point of the DFT band calculations the 77 and 5x5 orbital blocks corresponding to the one-electron 𝑉𝐿𝐹 potential acting on the 4f and 5d states, respectively. We show in the left panel of Figure 2 the energy splitting of the 5d states of Eu2+ inside BaSnSi3O9 and for the GC and EC structures. A complete list of CF parameters is shown in Table S1 of the Supporting Information (SI). With all these components, we diagonalize 𝐻 and calculate the energy levels and eigenfunctions. From that, we can calculate the dipolar transition probabilities (DTP) between states with the dipole operator. The right panel of Figure 2 shows the energy levels of the different 4f7 and 4f65d manifolds. Similar results are found for BaHfSi3O9:Eu2+ and BaZrSi3O9:Eu2+, a comparison between the three cases is shown in Figure S1 (SI). For each structure, the lowest 4f65d state is lower in energy in the EC than in the GC, in agreement with the larger crystal field splitting, also shown in Figure 2, corresponding to the shorter bond lengths (Table 1). Figure 2. Calculated crystal field energy levels of the 5d states, relative to their barycenter (left) and multielectronic energy states (right) of BaSnSi3O9:Eu2+ in the GC and EC structures. The crystal field levels also indicate their parentage: blue and red lines are used for the 4f7 and 4f65d manifolds, respectively. The zero of the scale corresponds to the lowest state. COMMUNICATION The emission wavelength from Eu2+ corresponds to the energy difference between the lowest 4f65d state and the 4f7 ground state in the EC structure. We calculate an emission wavelength of 475 nm for both BaHfSi3O9:Eu2+ and BaZrSi3O9:Eu2+, in agreement with the experimental data.24,25 For BaSnSi3O9:Eu2+ we predict an emission wavelength of 488 nm. A relevant parameter for the use of a phosphor in a particular application is the width of the emission peak. In order to estimate this width, we estimate the Franck-Condon (FC) factors,33 that indicate the coupling between different vibrational modes of the GC and EC configurations. We simplify the problem by just considering one vibrational mode. A complete description of the calculation of the FC factors is given in the SI. The computed FC factors are depicted as vertical blue lines in Figure 3. The emission spectra can be simulated as the envelope of the discrete spectrum made of 𝐹 𝑛 values, obtaining a full width at half maximum (FWHM) of 48 nm for BaHfSi3O9:Eu2+, in good agreement with the experimental data that finds FWHM~50 nm.24 For BaZrSi3O9:Eu2+ we obtain a FWHM of 43 nm, somewhat below the experimental data of FWHM~55 nm.25 Finally, we predict a FWHM of 64 nm for BaSnSi3O9:Eu2+. To verify our predictions, we have synthesized pure and Eu2+ (5 mol-%) doped BaSnSi3O9. X-ray diffraction (XRD) results are given in Figure S2 (SI). It is known that BaTiSi3O9 shows blue luminescence upon UV excitation, that has been assigned to a charge transfer (CT) transition within the TiO6 octahedra.34 This blue luminescence was also observed in natural Ba(Sn,Ti)Si3O935 and synthetic lattices like BaSnSi3O9:Ti4+ and BaZrSi3O9:Ti4+.36 Synthetic, Ti-free BaZrSi3O9 luminesces with a maximum at 290 nm that was assigned to a CT process within the ZrO6 octahedra.37 It is conceivable that BaSnSi3O9 will show SnO6 CT luminescence, however, at room temperature no luminescence was observed by Fujiwara et al.36 Our measurements on an undoped BaSnSi3O9 samples confirm this finding. CT luminescence often exhibits low quenching temperatures and it might be completely vanishing at 300 K. Figure 4 shows excitation and emission spectra of (Ba0.95Eu0.05)SnSi3O9 recorded at 4 K and at room temperature, respectively. The excitation spectrum (green lines) shows a strong temperature dependence. At 4 K (dark green line) the band is considerably broader and shows structuring. The structure is caused by the various possible 4f65d1 energy levels with non-vanishing DTP from the ground state, as shown in Figure 3. At room temperature (light green line) broadening of the individual lines results in the appearance of an unstructured band. The excitation is also considerably narrower at room temperature as the high energy part vanishes. The reflection spectrum of the undoped sample (Figure 5, black line) indicates that the conduction band begins at 350 nm and overlaps the 5d states of Eu2+. At room temperature this overlap hinders efficient excitation of Eu2+ at these wavelengths. At 4 K less mixing of the 5d states and the conduction band occurs and Eu2+ can be excited in this spectral region. Additionally, at room temperature an incision in the excitation spectrum is found at 395 nm due to a competing 7F0→5L6 absorption from Eu3+ impurities. Figure 3. Calculated absorption (red curves) and emission (blue curves) spectra of BaMSi3O9:Eu2+. Vertical red lines indicate the DTP from the 4f7 ground state to the 4f65d manifold. Vertical blue lines indicate Franck-Condon factors from the lowest vibrational mode of the lowest 4f65d state to the different vibrational modes of the 4f7 ground state. Figure 4. Experimental excitation spectra (green lines) monitoring the 490 nm emission peak at 4 K (dark green line) and at room temperature (light green line) and emission spectra (blue lines) upon 366 nm excitation at 4 K (dark blue line) and at room temperature (light blue line) of (Ba0.95Eu0.05)SnSi3O9. COMMUNICATION Upon excitation at 366 nm, an emission band (blue lines) with a maximum at 490 nm can be observed, which is very close to the predicted value of 488 nm. The emission spectrum is narrower at 4 K (dark blue line) than at room temperature (light blue line). This behavior is commonly observed and is a result of decreased coupling with vibrational states. The FWHM at 4 K is quite smaller than our prediction, indicating that our simplified model with just one vibrational mode might be an oversimplification. Fortunately, just like for BaHfSi3O9:Eu2+and BaZrSi3O9:Eu2+, our prediction fits well with the emission spectra at room temperature. Furthermore, at room temperature emission lines of Eu3+ are found between 580 and 710 nm, originating from the 5D0 to 7F0-4 transitions. At 4 K the relative intensity of the Eu2+ emission is much stronger and the Eu3+ lines cannot be as easily discerned. The reason for this is the low thermal quenching temperature of the Eu2+ emission as discussed later in this publication. The Eu3+ impurities can be diminished by use of a reducing atmosphere during annealing. However, this caused a brownish body color and strong decrease in luminescence intensity. This is caused by the reduction of Sn4+ and consequently by the formation of oxygen vacancies. Figure 5. Reflectance spectrum of (Ba0.95Eu0.05)SnSi3O9 (red line) and BaSnSi3O9 (black line). The photoluminescence quantum yield upon 366 nm excitation has been determined and found to be 17% ± 5%. To investigate the reason for this low quantum efficiency, emission spectra were recorded at temperatures from 77 to 500 K. The emission intensity was integrated in the range of 430 to 675 nm, the integrals being plotted over temperature (see Figure 6(a)). The experimental data were fitted to the Mott formula, to obtain the activation energy for the quenching process, finding a Ea = (0.39 ± 0.02) eV value. This energy corresponds to a thermal quenching temperature T1/2 of 267 K. For BaZrSi3O9:Eu2+ a higher T1/2 of approximately 450 K was reported.22 This difference can be explained if thermal quenching in these materials is caused by photo-ionization. This is a quenching mechanism where a localized electron in an excited state is further excited to the conduction band and consequently delocalized. If the energy difference between the excited state and the bottom of the conduction band is small enough, thermal energy can suffice to ionize the Eu2+ ion. Therefore, this quenching mechanism usually plays a role in phosphors with a small band gap and high excitation energies, such as LaAlO3:Ce3+.38-40 The larger band gap of BaZrSi3O9 in comparison to BaSnSi3O9 apparently results in a larger distance between the bottom of the conduction band and the excited state of Eu2+ and thus in a higher thermal quenching temperature. Figure 6. (a) Emission integral of (Ba0.95Eu0.05)SnSi3O9 upon 366 nm excitation in dependence on the temperature. (b) Decay times of the 490 nm emission of (Ba0.95Eu0.05)SnSi3O9 upon 375 nm excitation at temperatures from 77 to 375 K. To verify that the observed emission of (Ba0.95Eu0.95)SnSi3O9 consists in Eu2+ luminescence, the decay curves of the 490 nm emission, upon 375 nm excitation, were recorded at temperatures from 77 to 375 K (see Figure S3 of SI). The decay is mono-exponential, the half -time decreasing strongly with increasing temperature. A mono-exponential decay curve indicates a rather low defect density and a very homogeneous distribution of defects and activator ions in the crystallites.41-45 The decay curves were fitted to obtain the decay times, which are plotted in Figure 6(b). The sigmoidal trend of the decay parameters is typical for thermal quenching of the luminescence. By fitting the decay as function of temperature, the activation energy is obtained: Ea = 0.44 eV ± 0.01 eV, corresponding to a thermal quenching temperature T1/2 of 272 K. This value is in very good agreement with that derived from the emission intensity. It can be concluded that the emission certainly arises from a single entity, being not a combination of processes, e.g. Eu2+ with CT or defects luminescence. The decay time at 77 K is around 1 µs, which is a typical value for Eu2+ luminescence. Therefore, it can be stated that the observed luminescence is originating from Eu2+ ions in the host. The Figure 7 shows the comparison between the predicted and experimental emission curves, their similarity validating our theoretical prediction. Figure 7. Comparison between the predicted and the experimental emission spectra of BaSnSi3O9:Eu2+. COMMUNICATION In summary, we have used computational methods to predict the luminescence properties of a new phosphor BaSnSi3O9:Eu2+, undertaking, a posteriori, the experimental check. The predicted emission wavelength (488 nm) is close to the experimentally retrieved value (490 nm). Besides, the predicted and measured FWHM and curve shapes are also extremely similar. The outlined methodology opens the door to the application of computational methods in the design of future lighting devices. Methods DFT calculations were performed with the VASP code.46,47 The PAW method was used to reproduce the interaction between the core electrons and the valence states.48,49 The exchange-correlation potential was written in the PBE approach.50 Simulations were performed in periodic supercells of 112 atoms, so interaction between Eu2+ images is negligible. The ground and excited configurations of Eu2+ were calculated by including the 4f electrons inside its core and fixing the number of 4f electrons to 7 and 6, respectively. Starting structures for the calculations were obtaiden from available experimental data of the hosts.24,25,51 Experimental Section The (Ba0.95Eu0.05)SnSi3O9 powder sample was synthesized via conventional solid state method. BaCO3 (99.8% Alfa Aesar), EuF2 (99.9% Alfa Aesar), SnO2 (99.9% Alfa Aesar) and nanoscale SiO2 (99.5% Merck) were weighed in stoichiometric ratios and ground in an agate mortar employing n-hexane as grinding medium. The powder was transferred to a corundum crucible and sintered in nitrogen (99.999% Westfalen Gas) atmosphere at 1350 °C for 16 hours. Undoped BaSnSi3O9 was prepared by the same synthesis but without the addition of EuF2. Excitation and emission spectra were recorded on an Edinburgh Instruments FLS920 spectrometer equipped with a Heraeus 450 W Xe arc lamp, mirror optics for powder samples and a cooled (-20 °C) singlephoton counting photomultiplier (Hamamatsu R2658P). The excitation spectra were corrected by use of a reference detector. The emission spectra were corrected with a correction file obtained from a tungsten incandescent lamp certified by the NPL (National Physics Laboratory, UK). Decay curves were recorded using the FLS 920 spectrometer equipped with a 375 nm laserdiode and a TCSPC card. Temperature dependent ,measurements were conducted using the same FLS920 spectrometer with an Oxford Instruments MicrostatN2 sample holder and liquid nitrogen as cooling agent. XRD patterns were recorded on a Rigaku Miniflex II with Cu Kα1,2 radiation. Acknowledgements FC notes the support of UEFISCDI grant PCE 108/2017. FB and TJ want to thank Merck KGaA, Germany for financial support. Keywords: Density functional calculations • Ab initio calculations • Luminescence • Lanthanides [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, 1997. [2] T. Jüstel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 1998, 27, 2084. [3] J.-C. Bünzli, C. Piguet, Chem. Soc. Rev. 2005, 34, 1048. [4] H. A. Höppe, Angew. Chem. Int. Ed. 2009, 48, 3572. [5] X. Qin, X. Liu, W. Huang, M. Bettinelli, X. Liu, Chem. Rev. 2017, 117, 4488. [6] C. W. E. van Eijk, Phys. Med. Biol. 2002, 47, R85. [7] C. Ronda, Progress in Electromagtenic Research, 2014, 147, 81. [8] P. Pinho, K. Jokinen, L. Halonen, Lighting Res. Technol. 2012, 44, 427. [9] H.-Q. Wang, M. Batenschuk, A. Osvet, L. Pinna, C. J. Bravee, Adv. Mater. 2011, 23, 2675. [10] N. Kunkel, P. Goldner, Z. Anorg. Allg. Chem. 2018, 644, 66. [11] L. van Pieterson, M. F. Reid, G. W. Burdick, Phys. Rev. B 2002, 65, 045114. [12] P. S. Peijzel, P. Vergeer, A. Meijerink, M. F. Reid, L. A. Boatner, G. W. Burdick, Phys. Rev. B 2005, 71, 045116. [13] A. Canning, A. Chaudhry, R. Boutchki, N. Grønbech-Jensen, Phys. Rev. B 2011, 83, 125115. [14] M. Krośnicki, A. Kdziorski, L. Seijo, Z. Barandiarán, J. Phys. Chem. A 2014, 118, 358. [15] H. Ramanantoanina, W. Urland, A. García-Fuente, F. Cimpoesu, C. Daul, Chem. Phys. Lett. 2013 588, 260. [16] H. Ramanantoanina, W. Urland, A. García-Fuente, F. Cimpoesu, C. Daul, Phys. Chem. Chem. Phys. 2014, 16, 14625. [17] C. Bulloni, A. García-Fuente, W. Urland, C. Daul, Phys. Chem. Chem. Phys. 2015, 17, 24925. [18] B. Herden, A. García-Fuente, H. Ramanantoanina, T. Jüstel, C. Daul, W. Urland, Chem. Phys. Lett. 2015, 620, 29. [19] A. García-Fuente, A. Vega, Phys. Chem. Chem. Phys. 2017, 19, 15503. [20] G. Sánchez-Sanz, L. Seijo, Z. Barandiarán, J. Chem. Phys., 2010, 133, 114509. [21] M. de Jong, D. Biner, K. W. Krämer, Z. Barandiarán, L. Seijo, A. Meikerink, J. Phys. Chem. Lett., 2016, 7, 2730. [22] M. De Jong, A. Meikerink, L. Seijo, Z. Barandiarán, J. Phys. Chem. C, 2017, 121, 10095. [23] D. Xu, W. Chen, M. Zeng, H. Xue, Y. Chen, X. Sang, Y, Xiao, T. Zhang, R. R. Unocic, K. Xiao, L. Fu, Angew. Chem. Int. Ed. 2018, 57, 755. [24] D.-Y. Wang, Y.-C. Wu, T.-M. Chen, J. Mater. Chem. 2011, 21, 18261. [25] D.-Y. Wang, C.-H. Huang, Y.-C. Wu, T.-M. Chen, J. Mater. Chem. 2011, 21, 10818. [26] J. Y. Park, K. S. Shim, J. S. Yu, H. K. Yang, Mat. Lett. 2016, 173, 68. [27] Z. Barandiarán, L. Seijo, J. Chem. Phys., 2003, 119, 3785. [28] R. Valiente, F. Rodríguez, J. González, H. U. Güdel, R. MartínRodríguez, L. Nataf, M. N. Sanz-Ortiz, K. Krämer, Chem. Phys. Lett., 2009, 481, 149. [29] S. Mahlik, K. Wisniewsky, M. Grinberg, R. S. Meltzer, J. Phys. Condens. Matter, 2009, 21, 245601. [30] A. García-Fuente, F. Cimpoesu, H. Ramanantoanina, B. Herden, C. Daul, M. Suta, C. Wickleder, W. Urland, Chem. Phys. Lett., 2015, 622, 120. [31] R. D. Cowan, The Theory of Atomic Structure and Spectra, University of California Press, Ltd, Berkeley, 1981. [32] M. V. Putz, F. Cimpoesu, M. Ferbinteanu, Structural Chemistry, Principles, Methods and Case Studies, Springer, 2018. [33] B. Henderson, G. F. Imbusch, Optical spectroscopy of inorganic solids, Clarendon Press, Oxford, 1989. [34] M. Gaft, L. Nagli, G. Waychunas, D. Weiss, Phys. Chem. Miner. 2004, 31, 365. [35] E. B. Gross, J. E. N. Wainwright, B. W. Evans, Am Mineral. 1965, 50, 1164. [36] K. Iwasaki, Y. Takahashi, H. Masai, T. Fujiwara, Opt. Express 2009, 17, 18054. [37] G. Blasse, A. Bril, J. Solid State Chem. 1970, 2, 105. [38] B. Moine, C. Pedrini, B. Courtois, J. Lumin., 1991, 50, 31. [39] M. Raukas, S. A. Basun, W. van Schaik, W. M. Yen, U. Happek, Appl. Phys. Lett., 1996, 69, 3300. [40] E. van der Kolk, J. T. M. De Haas, A. J. J. Bos, C. W. E. Van Eijk, P. Dorenbos, J. Appl. Phys., 2007, 101, 083703. [41] H. Ni, H. Liang, Q. Su, Y. Tao, Y. Huang, Z. Gao, J. Am. Ceram. Soc. 2012, 95, 3197. [42] C. Liu, Z. Xia, Z. Lian, J. Zhou, Q. Yan, J. Mater. Chem. C, 2013, 1, 7139 COMMUNICATION [43] J. Qin, H. Zhang, B. Lei, C. Hu, J. Li, Y. Liu, J. Meng, J. Wang, M. Zheng, Y. Xiao, J. Am. Ceram. Soc., 2013, 96, 3149. [44] J. Chen, Y. Liu, L. Mei, H. Liu, M. Fang, Z. Huang, 2015, Sci. Rep. 5, 9673. [45] D. Cui, Q. Xiang, Z. Song, Z. Xia, Q. Liu, J. Mater. Chem. C, 2016, 4, 7332. [46] G. Kresse, J. Hafner, Phys. Rev. B 1993, 47, 558. [47] G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169. [48] P. Blöchl, Phys. Rev. B 1994, 50, 17953. [49] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758. [50] J. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. [51] J. Choisnet, A. Deschanvres, B. Raveau, J. Solid State Chem. 1972, 4, 209. COMMUNICATION Entry for the Table of Contents COMMUNICATION Computational methods and quantum mechanics are used to predict the properties of a brand new phosphor based on Eu(II). Our prediction is confirmed by later experiments. Author(s), Corresponding Author(s)* Page No. – Page No. Title