Crystal Chemistry and Dehydrogenation/Rehydrogenation Properties of Perovskite Hydrides RbMgH 3 and RbCaH 3

Crystal structure, lattice dynamics, and bonding environments of RbMgH 3 and RbCaH 3 were investigated using neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. RbMgH 3 exhibits a 6H-BaTiO 3 -type hexagonal perovskite structure, and RbCaH 3 forms a Pm-3m simple cubic perovskite. A very short Mg−Mg distance (∼2.77 Å) was found in RbMgH 3, and its structural stability was ascribed to H − anion polarization and Mg−Mg bonding. Interestingly, RbCaH 3 forms an ideal cubic perovskite despite its small tolerance factor. The inconsistency between the nominal tolerance factor and adopted structure types of RbCaH 3 are discussed in terms of anion polarization and covalence effect on the cation−anion bond length deviations. Finally, we show that both RbMgH 3 and RbCaH 3 can be dehydrogenated and rehydrogenated at 300−400 °C under moderate pressures.