Borate Hydrides as a New Material Class: Structure, Computational Studies, and Spectroscopic Investigations on Sr5(BO3)3H and Sr5(11BO3)3D

Abstract The unprecedented borate hydride Sr5(BO3)3H and deuteride Sr5(11BO3)3D crystallizing in an apatite‐related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+, it shows broad‐band orange‐red emission under violet excitation owing to the 4f65d–4f7 transition of Eu2+. The observed 1H NMR chemical shift is in good agreement with previously reported 1H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1H, 2H, natB, and 11B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange‐red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.


Structure analysis
: Refined lattice parameters and atomic parameters of the structural base model for Sr5( 11 BO3)3D (Pnma) at 300 K obtained from powder neutron diffraction data.   (7) 1.04(13) 1 Table S2: Additional refinement of lattice parameters and atomic parameters of the structural base model for Sr5( 11 BO3)3D0.92 (Pnma) at 300 K obtained from refinement of powder neutron diffraction data. Here, the occupancy for deuterium at the Wyckoff 4c site is refined.   Table S3. Crystal data and structure refinement of the two models Sr5( 11 BO3)3D and the hypothetical compound Sr5( 11 BO3)3OD against the same data set (Rietveld method, powder neutron diffraction data, T = 298 K). The hypothetical structure model Sr5( 11 BO3)3OD has been found to be invalid.

Empirical formula Sr5( 11 BO3)3D Sr5( 11 BO3)3OD
Neutron wavelength/ Å  Figure S1: Additional views on the crystal structure of Sr5( 11 BO3)3D. In the left view the hydride/deuteride anions run along the a axis. Right picture shows the view on the c axis. Figure S2: Coordination spheres of the three distinctive Sr-polyhedra. Atomic distances are given in Table S4. Sr1 is eight-fold coordinated by 7 oxygen and 1 hydrogen atom (SrO7H). The second coordination sphere of strontium (Sr2) comprises of 6 oxide anions and 1 hydride anion (SrO6H). Sr3 is surrounded by 9 oxide anions and can be described as a slightly distorted monocapped square antiprism that forms a double channel along the a-axis. Figure S3: Coordination spheres of oxygen centred polyhedra, that are necessary for understanding the photoluminescence emission spectra. Figure S4: Comparison of the simulated X-ray patterns with CuKa radiation for the compounds Sr5(BO3)3X (X = OH, H). The hydride pattern differs from the patterns of hydroxide the most at the 011 and 020 reflections. Angular range with significant differences is highlighted in green. The hypothetical crystal structure used for X = OH was obtained from a quantum chemical structure optimization.               Table S5 and S6. Two main vibrations occur in IR and Raman spectra: First, the black arrow depicts the movement that is perpendicular to the plane of the three strontium atoms forming a triangle, hereon called 'hydride out-of-plane mode' (2x Sr2, dark blue, and 1x Sr1, light blue). The other movement is parallel to the plane of the Sr3 triangle (white and red arrow), herein called 'hydride in-plane mode'.  From 50 cm -1 to about 320 cm -1 low energy lattice vibrations and borate rocking modes are observed (blue labels). At around 600 cm -1 , a B-O bending mode is visible. Little intensity in the experimental is observed for the borate out-of-plane bending, in agreement with theoretical expectations due to symmetry constraints. (Note, that these modes are described for the plane of the borate groups). At 910 cm -1 , breathing modes of the BO3 3groups show resonances in good agreement to the experiment. Similar to the IR spectra, we do not discuss the details for the nat B -11 B exchange, since the changes in the vibrational energies are minor compared to the hydride-deuteride shift.

Elemental analysis
Elemental analysis has been conducted on a Vario El microanalyzer with the Sr5( 11 BO3)3D sample and its deuterium content amounts to 0.26 wt%. Full occupation of the 4c Wyckoff site is assumed for D. The experimentally determined deuterium content is close to the theoretical value of 0.32 wt%. Figure S19: Elemental analysis of Sr5( 11 BO3)3D.

Full computational details
The solid-state NMR shielding tensors of Sr5(BO3)3H and hypothetical Sr5(BO3)3OH were calculated with the DFT-PBE method [2]. using the CASTEP program package and the GIPAW formalism as implemented in CASTEP-NMR [3] . Ultrasoft pseudopotentials generated with the on-the-fly scheme [4] and a planewave basis set cut-off of 630 eV were applied. The reciprocal space was sampled using a 3×2×3 Monkhorst-Pack-type k-mesh. [5] The NMR shielding tensor of Sr5(BO3)3H was calculated both at the experimental geometry and DFT-PBE optimized geometry. In the geometry optimization, both the lattice parameters and atomic positions were fully optimized with a total energy convergence criterion of 0.5 x 10 -5 eV/atom. The optimized lattice parameters a, b, and c of Sr5(BO3)3H differed from the experimental parameters by +1.1%, -0.4%, and +0.2%, respectively. The NMR shielding tensor of the hypothetical Sr5(BO3)3OH was calculated at the DFT-PBE0/TZVP-optimized geometry (see below). Molecular SiMe4 was used as a reference system for calculating the 1 H NMR shifts. The calculations on SiMe4 were carried out in a primitive cubic cell (a = 15 Å) using a plane-wave basis set cut-off of 700 eV and Γ-point for reciprocal space sampling. The structure of the SiMe4 molecule was relaxed within the Td point group. The isotropic 1 H shielding of SiMe4 is 31.01 ppm. The isotropic 1 H shielding of the hydride in Sr5(BO3)3H is 25.09 ppm at the experimental geometry and 25.10 ppm at the optimized geometry. Both values lead in identical 1 H chemical shift of 5.9 ppm. The isotropic 1 H shielding in the hypothetical Sr5(BO3)3OH (P212121) is 27.08 ppm, leading in a 1 H chemical shift of 3.9 ppm.
We also investigated Sr5(BO3)3H, Sr5( 11 BO3)3D, and hypothetical Sr5( 11 BO3)3OH using the CRYSTAL17 program package. [6] PBE0 hybrid density functional method and Gaussian-type basis sets were used. [7] The basis sets for Sr, O, and H have been previously derived from the molecular Karlsruhe def2 basis sets. [8] The basis set used for B is described in detail below. Polarized triple-zeta-valence (TZVP) basis sets were used for H, O, and B, polarized split-valence basis set for Sr. [9] The reciprocal space was sampled using a 4×2×3 Monkhorst-Pack-type k-mesh. [5] For the evaluation of the Coulomb and exchange integrals (TOLINTEG), tight tolerance factors of 8,8,8,8, and 16 were used. Both the atomic positions and lattice constants were fully optimized within the constraints imposed by the space group symmetry. In the case of Sr5(BO3)3H, the optimized lattice parameters a, b, and c differed from the experimental parameters by 0.0%, -0.3%, and -0.2%, respectively. The harmonic vibrational frequencies and IR intensities were obtained by using the computational schemes implemented in CRYSTAL.
[10] Sr5(BO3)3H and Sr5(BO3)3D were confirmed to be true local minima with no imaginary frequencies. Hypothetical Sr5(BO3)3OH showed imaginary frequencies when optimized in the space group Pnma. Distorting the geometry along the first imaginary mode (92i cm -1 ) decreased the symmetry to P212121. In this space group, the optimized structure is a true local minimum (full structural details report below).
The wavenumbers of the IR and Raman spectra have been scaled by a factor of 0.96 to account for the overestimation typical for predicted harmonic frequencies (see e.g. A. P. Scott, L. Radom J. Phys. Chem. 1996, 100, 16502-16513 and http://cccbdb.nist.gov/). The final IR spectra were obtained by using Lorentzian peak profile with FWHM of 8 cm −1 . The Raman intensities have been calculated for a polycrystalline powder sample (total isotropic intensity in arbitrary units). When simulating the Raman spectrum, the temperature and laser wavelength were set to values corresponding to the experimental setup (T = 298.15 K, λ = 532 nm). The final spectrum was obtained by using pseudo-Voigt peak profile (50:50 Lorentzian: Gaussian) and FWHM of 8 cm -1 .