title: Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites.
abstract: The structural phase transitions occurring in a series of perovskite-type complex hydrides based on the tetrahydroborate anion BH4(-) 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 BH4 anion are followed with quasi-elastic neutron scattering. We show that unexpected temperature-induced lattice instabilities in perovskite-type ACa(BH4)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 BH4(-) <-> (X = Halide) can relax distortions in ACa(BH4)3 by eliminating coulomb repulsive H(-)···H(-) effects. The interesting nature of phase transition in ACa(BH4)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.
authors.affiliation: Department of Quantum Matter Physics, Laboratory of Crystallography, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland.
authors.forename: P
authors.initials: P
authors.lastname: Schouwink
authors.forename: H
authors.initials: H
authors.lastname: Hagemann
authors.forename: J P
authors.initials: JP
authors.lastname: Embs
authors.forename: V
authors.initials: V
authors.lastname: D'Anna
authors.forename: R
authors.initials: R
authors.lastname: Černý