Ionic Mixed Hydride Fluoride Compounds: Stabilities Predicted by DFT

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Ionic Mixed Hydride Fluoride Compounds: Stabilities Predicted by DFT, Synthesis and Luminescence of Divalent Europium Nathalie Kunkel, and Holger Kohlmann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00386 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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The Journal of Physical Chemistry

Ionic Mixed Hydride Fluoride Compounds: Stabilities Predicted by DFT, Synthesis and Luminescence of Divalent Europium Nathalie Kunkel, ‡,†,* Holger Kohlmann# ‡

PSL Research University, Chimie Paristech-CNRS, Institut de Recherche de Chimie Paris

11, rue Pierre et Marie Curie, 75005 Paris, France. †

University of Saarland, Department of Chemistry, 66123 Saarbrücken, Germany

#

Inorganic Chemistry, University of Leipzig, Johannisallee 29, 04103 Leipzig, Germany

KEYWORDS. Hydride, fluoride, mixed crystals, europium luminescence, first principle calculations

ACS Paragon Plus Environment

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ABSTRACT. The hydride fluoride solid solution series LiSrH3-xFx. LiBaH3-xFx, KMgH3-xFx and EAH2-xFx (EA = Ca, Sr, Ba) were studied by first principle calculations and luminescence of SrH0.5F1.5:Eu2+ and KMgHF2:Eu2+ was observed. Theoretical calculations suggest the existence of complete solid solution series in case of LiBaH3-xFx and KMgH3-xFx, whereas only a partial solid solution series for the hydrogen-rich side is predicted for LiSrH3-xFx. For the luminescence of divalent europium in SrH0.5F1.5 an emission maximum at 600 nm is observed, which lies in between the emission wavelength of the pure hydride (728 nm) and fluoride (416 nm). Likewise, the emission maximum of Eu2+ in KMgHF2 was found to be approximately 505 nm, which corresponds to a blueshift relative to the hydride (565 nm) and a strong redshift compared to the fluoride (363 nm and ff emission). Consequently, substitution of hydride and fluoride shows a strong influence on the europium d levels due to the different polarizabilities.

Introduction Already in 1920 hydride was reported to show a halide-like behaviour1 and later the formation of a solid solution in the system LiH – LiF was observed2, 3. The observation that ionic compounds of hydride and fluoride may show structural analogies4, 5 can be related to the fact that the ionic radii of hydride and fluoride may be very similar in some structures. While the effective ionic radius of fluoride (for coordination number 6) is reported to be 133 pm6, hydride shows a high polarizability and sensitivity to the particular cation, resulting in variations of its effective radius from 127 to 153 pm6,7. For instance, the alkaline earth metal hydrides and fluorides show structural analogies5. While the hydrides of calcium, strontium and barium crystallize in the PbCl2 structure type at room temperature (space group Pnma) and transform


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into the cubic fluorite structure type at high temperatures, the fluorides crystallize in the cubic structure at ordinary pressures, but transform into the PbCl2 structure at higher pressures. Such temperature and pressure driven phase transitions are well-known for other alkaline earth halides as well8. Partial solid solution series, e.g. of the hydrides and fluorides of the earth alkaline metals strontium and barium9 or perovskite-type compounds5 have already been prepared. Hydride chlorides on the other hand usually do not form solid solutions because of the larger difference in ionic radii, but distinct stoichiometric compounds such as MHCl10, 11, M2H3Cl12 or M7H12Cl213. Due to the low shielding of the outer 5d electrons and the resulting sensitivity towards the chemical environment and local symmetry, Eu2+ luminescence can serve as a sensitive probe for the local coordination environment. Eu2+ emission has been reported for the fluorides CaF2, SrF2 and BaF2 (424, 416 and 403 nm14), LiBaF3 (ff emission15) and KMgF3 (363 nm and ff emission16), as well as the hydrides LiSrH3 (570 nm) and LiBaH3 (530 nm17), KMgH3 (565 nm18) and the alkaline earth metal hydrides CaH2, SrH2 and BaH2 (764, 728 and 750 nm19). The only study on Eu2+ luminescence in hydride fluoride solid solutions has only been carried out recently20. Besides, studies on rare-earth doped fluoride crystals with extremely small hydride doping (hydrogen or deuterium introduced by heating samples in hydrogen atmosphere and in contact with molten aluminum) were reported21. In the present paper we study Eu2+ luminescence in a mixed strontium hydride fluoride as well as the perovskite-type mixed anion compound KMgHF2 and discuss the stability of hydride fluoride solid solution series in some perovskite type compounds by means of density functional calculations.


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Materials and Methods The strontium hydride fluorides were obtained via solid state reactions of the binary hydrides and fluorides under hydrogen pressure in a hydrogen resistant Nicrofer® 5219 autoclave (Inconel 718) at 740 K and 30 bar H2 pressure (99.9%) for 3 days. Strontium fluoride was purchased from ABCR (99.99%) whereas europium doped strontium hydride was obtained via hydrogenation of a europium strontium alloy (metals mechanically surface cleaned, strontium Alfa Aesar, 99.9%, europium Alfa Aesar 99.9%). Despite of surface cleaning, the metals, and especially strontium, sometimes contain small amounts of oxide that are difficult to remove. KMgHF2:Eu2+ (0.5 mol%) was prepared via a solid state reaction of MgF2 (ABCR 99.99%), KH (Aldrich, 50 wt% in paraffin, washed several times with hexane) and EuH2 (obtained by hydrogenation of europium metal) in a silica pressure cell with an indium seal at 11 bar and 920 K (4 hours). Attempts to synthesize LiSrF3 from the binary fluorides LiF (Merck 99%) and SrF2 (ABCR 99.99%) were carried out at 900-1100K under argon or in vacuum and LiSrHF2 from LiH (Acros Organics 98%) and SrF2 in a silica pressure cell under 10 bar H2 at 950 K. All metals, hydrides and hydride fluorides were handled in an argon-filled glove box. Special care has to be taken when handling devices under high pressure and/or temperature. Mechanical shielding for the case of disintegration due to mechanical failure, which can be extremely harmful, is mandatory. Samples were characterized by X-ray powder diffraction on a Bruker D8 Advance powder diffractometer (with Lynxeye) with Bragg-Brentao geometry and a fine focus X-ray tube


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(CuKα1,2 radiation) or using flat transmission samples on a Huber Guinier G670 camera with an image plate system (Cu-Kα1 radiation). Due to moisture and air-sensitivity, samples were enclosed between kapton foils in apiezon grease. Data were collected for 60 min in the diffraction range 10 – 110° 2θ. Crystal structures were refined via the fundamental parameter approach22 using TOPAS 4.2 (Bruker AXS, Karlsruhe, Germany)23. In order to determine the instrumental function, reference scans of LaB6 and Si were used. Photoluminescence emission and excitation spectra were measured on an Edinburgh Instruments FLS920 spectrofluorometer. The spectrometer was equipped with a 450 W xenon lamp for excitation, a double monochromator according to Czerny-Turner (300 nm focal length) for the excitation beam and a single monochromator and a photomultiplier tube (R928P Hamamatsu) for UV/Vis detection. Spectra were corrected for lamp intensity and photomultiplier sensitivity. Samples were enclosed in sealed silica tubes of 1 cm diameter. Luminescence decay curves of SrF1.5H0.5:Eu2+ were measured under pulsed excitation with an Edinburgh 376.8 nm pulsed diode laser (65 ps pulses). Electronic structure calculations were performed with the Vienna ab initio simulation package (VASP 5.3)24, 25 together with PAWs26. Semi core states were taken into account for all metal potentials (sv). Thus, the potentials contained 10 valence electrons for barium, 10 for calcium, 7 for fluorine, 1 for hydrogen, 9 for potassium, 3 for lithium, 10 for magnesium and 10 for strontium. Exchange correlation effects were treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof revised for solids (PBEsol)27. The tetrahedron method with Blöchl corrections28 was used to evaluate the electronic properties on a Gamma-centered grid (8 x 8 x 8 for the perovskite supercells, 15 x 15 x 15 for the LiH, LiF, KH, KF, SrF2 and BaF2 unit cells, 10 x 16 x 8 for the PbCl2-type unit cells, 12 x 12 x 16 for MgH2 and MgF2 in the rutile structure


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type, 4 x 16 x 8 for the 3 x 1 x1 PbCl2-type super cells and 8 x 8 x8 for the fluorite-type super cells). Structural examples for the inverse perovskite type, the fluorite type and the PbCl2-type are given in the Supporting Information for LiBF3, BaF2 and BaH2. The cut-off for the planewave expansion of the electronic orbitals was set to 600 eV. Exemplary, we also tested a cut-off of 700eV together with hard potentials for hydride and fluoride (h), but did not obtain significantly different results. Due to the much higher computational requirements we did not continue these calculations. All lattice parameters as well as free positional parameters were allowed to relax. The electronic energies were converged to 0.01 meV and the forces to 0.001 meV/pm.

Results and Discussion The solid state reaction of the corresponding amounts of the binary strontium hydrides and fluorides under hydrogen pressure led to the compounds with the nominal composition SrH0.5F1.5:Eu2+ (0.125 mol%) and SrH0.2F1.8: Eu2+ (0.5 mol%) in the fluorite structure type with small amounts of the impurity SrO, which, despite of surface cleaning, probably originates from the strontium metal used in the synthesis (see the Rietveld refinement for x = 1.5 in Fig.1). Formation of very small amounts of EuO can therefore not be completely excluded. In case of x = 1.5, Eu2+-doped SrH2 was used, whereas for x = 1.8 EuH2 was added to the mixture of SrH2 and SrF2. The refined lattice parameters are given in Table 1. Since the occupation numbers for hydride and fluoride cannot be refined to satisfactory precision, the given x value is only a nominal value.


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Table 1. Refined lattice parameters and impurity phases of the samples used for luminescence spectroscopy. Compound

a [pm]

SrH0.5F1.5:Eu2+(0.125 mol%) SrH0.2F1.8:Eu2+ (0.5 mol%) KMgHF2:Eu2+ (0.5 mol%)

581.115(5)

Impurity phases [w%] 4.15(9) SrO

580.050(2) 399.241(6)

2.2(1) SrO 8.9(1) MgF2

Figure 1. Rietveld analysis of the X-ray powder diffraction data of SrH0.5F1.5:Eu2+ (approx. 0.125 mol%).

A weak orange-reddish luminescence emission was observed for x = 1.5 with an emission maximum at approx. 600 nm (see Fig.2). This corresponds to a blueshift of more than 100 nm relative to the emission maximum of the pure hydride SrH2:Eu2+ (728 nm) and a strong redshift of almost 200 nm relative to the pure fluoride SrF2:Eu2+ (416 nm). The broad emission band together with the slight shoulder on the low energy side of the band suggests that the hydride and fluoride concentrations in the local environments of the Eu2+ ions are distributed inhomogeneously in the host lattice.


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Figure 2. Luminescence excitation and emission spectra of SrH0.5F1.5:Eu2+ (approx. 0.125 mol%) at room temperature.

Decay time measurements of SrH0.5F1.5:Eu2+ (0.125 mol%) showed a biexponential decay with τ = 25 ns (80%) and 170 ns (20%). This is in agreement with the observation that the emission band seems to originate from a distribution of different local environments with different hydride concentrations. Such a situation might lead to energy transfer via non-radiative processes between the different centers that might cause the biexponential decay. However, it is also known that the quenching temperature of Eu2+ in hydrides is often below room temperature17. Obtaining a complete understanding of the non-radiative processes will require a detailed temperature-dependent study with a larger number of europium concentrations. No emission was observed for SrH0.2F1.8:Eu2+ (0.5 %mol), which might be caused by concentration quenching due to the higher europium concentration. Another possibility is that EuH2 was not incorporated into the host lattice, even though the diffraction data did not show any EuH2 impurity.


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The reaction of MgF2, KH and EuH2 yielded a perovskite-type compound (normal perovskite) as main phase together with about 8.9(1) w% MgF2 as an impurity phase. The true composition might therefore be slightly richer in hydrogen as compared to the nominal KMgHF2. The lattice parameter a of the perovskite was refined to be a 399.241(6) pm. This is larger than the pure fluoride (KMgF3 398.35 pm29 and much smaller than for the pure hydride KMgH3 (402.869(4) pm, prepared by the authors). The compound showed a green luminescence emission upon excitation at 360 nm with a maximum of the emission band at about 505 nm. This corresponds to a blueshift of about 60 nm relative to the hydride and a strong redshift compared to the fluoride (363 nm and ff emission16). Thus, as expected, substitution of hydride by fluoride has a strong influence on the position of the europium d levels and therefore on the emission energies.

Figure 3. Rietveld analysis of the X-ray powder diffraction data of KMgHF2:Eu2+ (0.5 mol%). Even though the hypothetical compound LiSrF3 has a tolerance factor within the limit of the necessary values for the perovskite formation5, it did not form from LiF and SrF2 at 900-1100 K. We explain this observation with the thermodynamic instability of the compound relative to the binary fluorides (for more explanation, see the results of our theoretical calculations, e.g. Fig. 4). Likewise, attempts to synthesize LiSrHF2:Eu2+ in a silica pressure cell at 10 bar H2 pressure and


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temperatures up to 950 K from the binary hydrides and fluorides as well as from LiSrH3:Eu2+ and the binary fluorides were not successful. Such reactions yielded only small amounts of a perovskite phase (inverse perovskite) with lattice parameters around 383-384 pm (close to the pure hydride) together with large amounts of binary hydrides and fluorides. This suggests that a perovskite phase with lower fluoride content is formed. Clarification of this question will require further studies, possibly at even higher temperatures. Some grains in these samples usually showed a very weak yellow luminescence, whose origin is unclear and which may probably be attributed to a hydrogen-rich perovskite phase.

For the theoretical calculations the solid solutions of the perovskite structures were modelled through 3 x 3 x 3 supercells in space group Pm-3m. As clarified using neutron powder diffraction17, 30 and refined by the authors, LiSrH3, LiBaH3 and LiBaF3 crystallize in the inverse cubic perovskite structure type and their solid solution series were therefore also modelled in the inverse perovskite type. KMgH3 and KMgF331, in contrast crystallize in the normal cubic perovskite structure type. For KMgH3, this was shown by theoretical calculations18. The supercells were obtained by symmetry reduction starting from the unit cell in Pm-3m via a isomorphic transition with tripling of the axes. The resulting sites were 1a, 6e, 12i and 8g for lithium or magnesium, 8g, 12j, 6f and 1b for strontium, barium or potassium and 3d, 6e, 12h, 12j, 24k and 24m for hydrogen or fluorine. The mixed crystals of the binary hydrides and fluorides were carried out using supercells of the fluorite and PbCl2 structure type. The fluorite-type supercell was obtained starting from the fluorite structure in Fm-3m via a klassengleiche (IIa) transition to Pm-3m followed by a k2 with doubling of the axes to Fm-3m and a k4 to Pm-3m, Sr 1a, 3d, 3c, 1b, 12i, 12j and F/H 8g, 8g, 24m, 24m. For the PbCl2 structure a 3 x 1 x1 supercells


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was used. Tripling of the a axis (isomorphic transition) leads to 3 times 4c for strontium and 6 times 4c for hydride or fluoride.

Figure 4. Enthalpies of formation for the perovskite-type hydride fluoride solid solutions calculated with the Vienna ab-initio program code.

Fig. 4 shows the enthalpies of formation for the perovskites LiSrH3-xFx, LiBaH3-xFx and KMgH3xFx,

calculated for the reaction of the binary hydrides and fluorides, for example LiH + SrH2 →

LiSrH3. In cases where more than one combination of hydride and fluoride was possible, the one with the lowest total energy for the mixture was chosen. Note that entropy is not taken into account, since the calculations are only valid for 0 K. Furthermore, due to the use of super cells instead of statistically distributed hydride and fluoride small errors may be introduced as a consequence of errors in the mixing entropy. According to the calculations both solid solution series, LiBaH3-xFx and KMgH3-xFx are predicted to be stable for the whole x range. In contrast,


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the solid solution LiSrH3-xFx is only stable in a limited range for x. Already for x around 0.75 the compounds become unstable, even though the real value is probably slightly higher, since the mixing entropy is supposed to also play a role. This is in good agreement with the experimental observation that it was not possible to obtain LiSrHF2. The errors in the calculated lattice constants compared to known experimental values are approximately 1%. Thus, we assume the errors to be similar also for those compounds that have not yet been experimentally characterized. The predicted stability of the (partial) solution series suggest that it should in principle also be possible to experimentally obtain a continuous variation in the emission wavelength by doping with divalent europium or possibly another luminescence center with a df transition involved. Table 2. Calculated formation enthalpies and lattice parameters of the unit cell for LiSrH3xFx. x 0 0.11 0.33 0.77 1.22 2.11 2.33 2.67 3

EF (KJ/mol) -16.5 -13.6 -9.3 +0.7 +12.6 +25.3 +28.6 +29.0 +29.8

a (pm) 375.57 375.82 376.60 377.51 378.17 379.84 379.93 380.36 380.61

Table 3. Calculated formation enthalpies and lattice parameters of the unit cell for LiBaH3xFx. x 0 0.11 0.33 0.77 1.22 2.11 2.33 2.67

EF (KJ/mol) -32.44 -30.51 -27.91 -21.35 -12.71 -5.11 -1,99 -2.06

a (pm) 395.82 395.94 396.10 396.45 396.61 396.77 397.00 397.02


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3

-1.83

397.10

Table 4. Calculated formation enthalpies and lattice parameters of the unit cells for KMgH3-xFx. x 0 0.11 0.33 0.77 1.22 2.11 2.33 2.67 3

EF (KJ/mol) -41.55 -41.15 -40.80 -39.90 -36.24 -37.69 -35.46 -40.12 -41.62

a (pm) 397.48 397.57 397.92 398.19 398.20 398.86 398.82 398.90 399.06

In Fig. 5 the enthalpies of formation for the solid solution series CaH2-xFx, SrH2-xFx and BaH2xFx,

calculated for the fluorite and the PbCl2 structure type are depicted. The enthalpies were

calculated assuming the fluoride AEF2 in the fluorite type and the hydride AEH2 in the PbCl2 structure type as starting compounds. Thus, all energies of formation are positive, because the mixing entropy is not correctly taken into account by the super cell approach. It may be assumed that all compounds with relatively small positive energies of formations could actually exist. In Tables 5-7 the energies of formation as well as the calculated lattice constants for the binary solid solution series CaH2-xFx, SrH2-xFx and BaH2-xFx are given. For M = Ca, the fluorite structure type already becomes energetically more stable for very small fluoride contents. In case of SrH2-xFx and BaH2-xFx the fluorite structure type becomes more stable for an x of about 0.95, and 0.9, respectively. Additionally, Fig. 6 depicts the volume per formula unit for both the PbCl2 and the fluorite structure type.


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Depending on the chosen Wyckoff site for substitution, increases or decreases of the different lattice parameters of the PbCl2 structure type can be observed which also leads to some small deviation from linear behavior of the volume with regard to x. However, it is clearly visible that the volume in the PbCl2 structure type is always significantly smaller than in the fluorite structure type. This and the higher polarizability of hydride compared to fluoride is also in agreement with the observation that the hydrides transform into the fluorite structure type at higher temperatures whereas the fluorides transform into the PbCl2 type at higher pressure5. Table 5. Calculated energies of formation and lattice parameters of CaH2-xFx in the fluorite and PbCl2 structure type. x 0 0.25 0.33 0.66 0.75 1 1.25 1.33 1.66 1.75 2

fluorite EF (KJ/mol) a (pm) 0.90 537.19 0.85 538.02 0.77 539.31 0.76 540.22 0.72 540.56 0.32 541.91 0 542.44

EF (KJ/mol) 0 10.48 22.40 42.80 40.60 51.61 52.28

a (pm) 520.14 564.28 573.42 556.28 573.59 571.59 548.90

PbCl2 b (pm) 378.53 373.73 360.61 373.40 349.49 401.57 406.29

c (pm) 654.70 642.41 659.54 644.57 707.31 638.48 645.20

Table 6. Calculated energies of formation and lattice parameters of SrH2-xFx in the fluorite and the PbCl2 structure type. x 0 0.25 0.33 0.66 0.75 1 1.25 1.33 1.66 1.75

fluorite EF (KJ/mol) a (pm) 12.76 580.22 11.15 579.31 7.79 577.87 6.40 577.96 4.66 576.98 1.62 576.71

EF (KJ/mol) 0 1.81 3.39 8.10 11.72 11.57 -

a (pm) 625.17 630.31 634.62 639.17 634.14 628.63 -

PbCl2 b (pm) 381.24 382.08 383.12 383.61 380.74 378.76 -

c (pm) 720.64 725.59 730.46 732.80 735.53 737.58 -


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2

0

576.50

10.53

625.45

376.66

739.09

Table 7. Calculated energies of formation and lattice parameters for BaH2-xFx in the fluorite and PbCl2 structure type. x 0 0.25 0.33 0.66 0.75 1 1.25 1.33 1.66 1.75 2

fluorite EF (KJ/mol) a (pm) 15.62 627.51 13.76 625.86 9.73 622.29 8.04 621.73 5.85 619.83 2.01 618.40 0 617.60

EF (KJ/mol) 0 2.87 5.34 10.82 12.94 10.71 8.47

a (pm) 669.76 670.02 666.78 665.53 665.95 666.14 667.03

PbCl2 b (pm) 412.05 404.85 400.16 396.32 398.96 401.01 402.86

c (pm) 776.34 779.14 780.14 780.43 785.07 788.69 791.40

Figure 5. Enthalpies of formation for the alkaline earth metal (M = Ca, Sr, Ba) hydride fluoride solid solutions calculated in the fluorite and the PbCl2 structure type.


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Figure 6. Volumes per formula unit for the alkaline earth metal (M = Ca, Sr, Ba) hydride fluoride solid solutions calculated in the fluorite and the PbCl2 structure type.

Conclusions Theoretical calculations of the solid solution series LiSrH3-xFx, LiBaH3-xFx, and KMgH3-xFx in the normal cubic perovskite or inverse cubic perovskite structure type predict that complete solid solution series exist for LiBaH3-xFx and KMgH3-xFx. In contrast, and in good agreement with experimental findings only a partial solid solution series of LiSrH3-xFx on the hydrogen rich side is predicted. This shows the similarity of hydride and fluoride in similar ionic structures, which is due to the polarizability of the hydride ion and its ability to adopt ionic radii very close to those of fluoride. The possibility to design the polarizability of the anion sub lattice by substitution of a hard ion by a rather soft ion and vice versa as well as the first luminescence experiments with divalent europium suggests that such systems allow the tailoring of the


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emission wavelengths. However, in order to obtain compounds with a defined degree of substitution and a good homogeneous distribution on order to avoid unnecessary luminescence quenching an improvement of the synthesis is clearly necessary. It might be worthwhile to carry out syntheses at higher temperatures and pressures. Furthermore stabilities of the earth alkaline metal hydride fluorides (Ca, Sr, Ba) were calculated and except in case of CaH2 in the PbCl2 structure type it seems likely that it is possible to substitute a certain amount of hydride by fluoride in the PbCl2 structures as well as vice versa in the fluorite structures. In case of a mixed strontium hydride fluoride crystallizing in the fluorite type and doped with divalent europium an emission wavelength in between that of the pure hydride and fluoride was observed, again supporting the idea that a tailoring of the emission wavelength by substitution of hydride by fluoride and vice versa is possible.

ASSOCIATED CONTENT Supporting Information. Structural pictures of the compounds under investigation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected], +33 1 53 73 79 23 Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Deutsche Forschungsgemeinschaft (DFG, grant no. KO 1803/7-1) Landesgraduiertenförderung Saarland (LGFG) ACKNOWLEDGMENT The authors would like to thank M. Springborg, N. Louis, S. Kohaut and C3M Saar for providing computational resources and technical support, A. Meijerink for help with the luminescence measurements, O. Oeckler to provide beamtime for collecting the XRD data of some of the samples and Ph. Goldner for useful discussion. Financial support of the Deutsche Forschungsgemeinschaft (DFG, grant KO1803/7-1) and the Landesgraduiertenförderung Saarland is gratefully acknowledged.

Dedicated to Prof. Horst P. Beck on the Occasion of His 75th Birthday

REFERENCES [1] Moers, K. Untersuchungen über den Salzcharakter des Lithiumhydrids. Z. Anorg. Allg. Chem. 1920, 113, 179-228. [2] Zintl, E.; Harder, A. Alkali hydrides. Z. Phys. Chem. B 1931, 14, 265-284. [3] Messer, C. E.; Mellor, J. The system lithium hydride – lithium fluoride. J. Phys. Chem. 1960, 65, 503-505.


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[4] Messer, C. E. Hydrides versus fluorides: Structural comparisons. J. Solid State Chem. 1970, 2, 144-155. [5] Maeland, A.J.; Lahar, W. D. The hydride-fluoride analogy. Z. Phys. Chem. 1993, 179, 181185. [6] Shannon, R.D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr., Sect. A: Found- Crystallogr. 1976, 32, 751-767. [7] Gibb, T. R. P. Jr.; Schumacher, D. P. Internuclear distances in hydrides. J. Phys. Chem. 1969, 64, 1407-1409. [8] Beck, H. P. Zur Hochdruckpolymorphie der Dihalogenide MX2 (M: Ca, Sr, Eu; X: F, Cl, Br). Z. Anorg. Allg. Chem. 1979, 459, 72-80. [9] Brice, J. – F.; Perrin, M.; Leveque, R. Hydrofluorures ioniques MF2-xHx (M = Sr, Ba): Synthèse et étude structurale par diffraction des neutrons. J. Solid State Chem. 1979, 30, 183188. [10] Beck, H. P.; Limmer, A. Die Verfeinerung der Kristallstrukturen von CaHCl, SrHCl, BaHCl, BaHBr und BaHCl. Z. Anorg. Allg. Chem. 1983, 502, 185-190. [11] Ehrlich, P.; Alt, B.; Gentsch, L. Über die Hydridchloride der Erdalkalimetalle. Z. Anorg. Allg. Chem. 1956, 283, 58-73. [12] Reckeweg, O.; Molstad, J. C.; Levy, S.; Di Salvo, F. J. Syntheses and crystal structures of the new ternary barium halide hydrides Ba2H3X (X = Cl or Br). Z. Naturforsch., B: Chem. Sci. 2007, 62, 23-27.


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[13] Reckeweg, O.; Molstad, J. C.; Levy, S.; Hoch, C.; Di Salvo, F. J. Syntheses and crystal structures of Sr7H12X2 (X = Cl, Br), Z. Naturforsch., B: Chem. Sci. 2008, 63, 513-518. [14] Kobayasi, T.; Mroczkowski, S.; Owen, J. F.; Brixner, L. H. Fluorescence lifetime and quantum efficiency for 5d-4f transitions in Eu2+ doped chloride and fluoride crystals. J. Lumin. 1980, 21, 247-257. [15] Meijerink, A. Spectroscopy and vibronic transitions of divalent europium in LiBaF3. J. Lumin. 1993, 55, 125-138. [16] Sommerdijk, J. L.; Bril, A. Divalent europium luminescence in perovskite-like alkalineearth alkaline fluorides. J. Lumin. 1976, 11, 363-367. [17] Kunkel, N.; Meijerink, A.; Kohlmann, H. Bright yellow and green Eu(II) luminescence and vibronic fine structures in LiSrH3, LiBaH3 and their corresponding deuterides. Phys. Chem. Chem. Phys. 2014, 16, 4807-4813. [18] Kunkel, N.; Meijerink, A.; Springborg, M.; Kohlmann, H. Eu(II) luminescence in the perovskite host lattices KMgH3, NaMgH3 and mixed crystals LiBaxSr1-xH3. J. Mater. Chem. C 2014, 2, 4799-4804. [19] Kunkel, N.; Kohlmann, H.; Sayede, A.; Springborg, M. Alkaline-earth metal hydrides as novel host lattices for EuII luminescence. Inorg. Chem. 2011, 50, 5873-5875. [20] Kunkel, N.; Meijerink, A.; Kohlmann, H. Variation of the EuII emission wavelength by substitution of fluoride by hydride in fluorite-type compounds EuHxF2-x (0.20 ≤ x ≤ 0.67). Inorg. Chem. 2014, 53, 4800-4802.


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[21] Reeves, R.J.; Jones, G. D.; Syme, R. W. G. Site-selective laser spectroscopy of Pr3+ C4v symmetry centers in hydrogenated CaF2:Pr3+ and SrF2:Pr3+ crystals. Phys Rev. B: Condens. Matter Mat. 1992, 46, 5939-5959. [22] Coelho, A. A. Indexing of powder diffraction patterns by iterative use of singular value decomposition. J. Appl. Crystallogr. 2003, 36, 86-95. [23] Bruker, AXS, Karlsruhe, TOPAS V4.2, Gerneral profile and structure analysis software for powder diffraction data, Users’s Manual, 2009. [24] Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B: Condens. Matter Mat. 1996, 54, 1116911186. [25] Hafner, J. Ab-Initio Simulations of materials using VASP: Density-functional theory and beyond. J. Comput. Chem. 2008, 29, 2044-2078. [26] Blöchl, P. E. Projector-augmented wave method. Phys Rev B: Condens. Matter Mat. 1994, 50, 17953-17979. [27] Perdew, J.P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Restoring the density-gradient expansion for exchange in solids and surfcaes. Phys. Rev. Lett. 2008, 100, 136406. [28] Blöchl, P. E.; Jepsen, O.; Andersen, O.K. Improved tetrahedron method for Brillouin-zone integrations. Phys Rev B: Condens. Matter Mat. 1994, 49, 16232-16233.


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[29] Mitchell, R. H.; Cranswick, L. M. D.; Swainson, I. Neutron diffraction determination of the cell dimensions and thermal expansion of the fluoropervoskite KMgF3 from 293 to 3.6 K. Phys. Chem. Min. 2006, 33, 587-591. [30] Maeland, A. J.; Andresen, A. F. Ternary hydrides possessing the cubic perovskite structure. I. A neutron-diffraction study of BaLiH3 and BaLiD3. J. Chem. Phys. 1968, 48, 46604661. [31] Schumacher, R.; Weiss, A. KMgH3 single crystals by synthesis from the elements. J. Less-Comm. Met. 1990, 163, 179-183.

TOC Entry The hydride fluoride solid solution series LiSrH3-xFx, LiBaH3-xFx, KMgH3-xFx and EAH2-xFx (EA = Ca, Sr, Ba) were studied by first principle calculations and luminescence of Eu(II) in SrH0.5F1.5 and KMgHF2 was observed.

TOC-Graphic


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Figure 1. Rietveld analysis of the X-ray powder diffraction data of SrH0.5F1.5:Eu2+ (approx. 0.125 mol%). 82x58mm (300 x 300 DPI)



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Figure 2. Luminescence excitation and emission spectra of SrH0.5F1.5:Eu2+ (approx. 0.125 mol%) at room temperature 82x58mm (300 x 300 DPI)


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Figure 3. Rietveld analysis of the X-ray powder diffraction data of KMgHF2:Eu2+ (0.5 mol%). 82x58mm (300 x 300 DPI)



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Figure 4. Enthalpies of formation for the perovskite-type hydride fluoride solid solutions calculated with the Vienna ab-initio program code. 82x117mm (300 x 300 DPI)


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Figure 5. Enthalpies of formation for the alkaline earth metal (M = Ca, Sr, Ba) hydride fluoride solid solutions calculated in the fluorite and the PbCl2 structure type. 82x117mm (300 x 300 DPI)



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Figure 6. Volumes per formula unit for the alkaline earth metal (M = Ca, Sr, Ba) hydride fluoride solid solutions calculated in the fluorite and the PbCl2 structure type. 82x117mm (300 x 300 DPI)


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Ionic Mixed Hydride Fluoride Compounds: Stabilities Predicted by DFT

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