Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites

The structural phase transitions occurring in a series of perovskite-type complex hydrides based on the tetrahydroborate anion ${\text{BH}}_4^{-}$ are investigated by means of in situ synchrotron x-ray powder diffraction, vibrational spectroscopy, thermal methods and ab initio calculations in the solid state. Structural dynamics of the $\text{B}{\text{H}}_4$ anion are followed with quasi-elastic neutron scattering. We show that unexpected temperature-induced lattice instabilities in perovskite-type ACa($\text{B}{\text{H}}_4$)₃ (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts. The rich lattice dynamics lead to coupling between internal B-H vibrations and phonons, resulting in distortions in the high-temperature polymorph that are identical in symmetry to well-known instabilities in oxide perovskites, generally condensing at lower temperatures. It is found that anion-substitution ${\text{BH}}_4^{-}$ $\leftrightarrow X^{-}$ (X = Halide) can relax distortions in ACa($\text{B}{\text{H}}_4$)₃ by eliminating coulomb repulsive ${\text{H}}^{-}\cdot \cdot \cdot {\text{H}}^{-}$ effects. The interesting nature of phase transition in ACa($\text{B}{\text{H}}_4$)₃ enters an unexplored field of weak interactions in ceramic-like host lattices and is the principal motivation for this study. Close di-hydrogen contacts suggest new concepts to tailor crystal symmetries in complex hydride perovskites in the future. (paper)