Phase Transitions in Borohydride Perovskites

A series of complex hydrides based on the highly dynamic tetrahydroborate anion BH₄^- and crystallizing in the ABX₃ type lattice has recently been discovered. They present a rare case of a family of iono-covalent hydrides that has a genuine tunable host lattice, making them an interesting new class of host compounds for not only the design of hydrogen storage materials but also hydride-properties related to heavy metals. Amongst these, preliminary results on REE-based luminescence will be discussed in the neat and doped compounds, the Ln²⁺ excited states surprisingly not being subject to significant quenching by B-H vibrations. Unlike oxide- or halide-perovskites some members of the AB(BH₄)₃ group do not evolve to higher symmetries as a function of temperature. We show by means of in-situ synchrotron X-ray powder diffraction, vibrational spectroscopy and ab initio calculations in the solid state, that temperature-induced structural distortions in perovskite-type ACa(BH₄)₃ (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts of repulsive nature. Coupling between internal B-H vibrations and phonons results in lattice distortions that are identical in symmetry to well-known instabilities (soft modes) in perovskites, which generally condense to lower temperatures. Anion-substitution BH₄^- <-> X^- (X = Halide) calculated on ordered models can relax distortions caused by repulsive effects. High temperature phase-transitions in ACa(BH₄)₃ can be of first or second-order, including 2-fold superlattices, simple cubic-cubic transitions accompanied by volume expansion or complex modulated superstructures accompanied by negative volume expansion, as is the case in RbCa(BH₄)₃. Close di-hydrogen contacts may be suggested as a tool to tailor the crystal symmetry in complex hydride perovskites in the future.