Crystal Chemistry and Dehydrogenation/Rehydrogenation Properties of Perovskite Hydrides RbMgH3 and RbCaH3

Abstract

Crystal structure, lattice dynamics, and bonding environments of RbMgH3 and RbCaH3 were investigated using neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), and first-principles calculations. RbMgH3 exhibits a 6H-BaTiO3-type hexagonal perovskite structure, and RbCaH3 forms a Pm-3m simple cubic perovskite. A very short Mg−Mg distance (∼2.77 A) was found in RbMgH3, and its structural stability was ascribed to H− anion polarization and Mg−Mg bonding. Interestingly, RbCaH3 forms an ideal cubic perovskite despite its small tolerance factor. The inconsistency between the nominal tolerance factor and adopted structure types of RbCaH3 are discussed in terms of anion polarization and covalence effect on the cation−anion bond length deviations. Finally, we show that both RbMgH3 and RbCaH3 can be dehydrogenated and rehydrogenated at 300−400 °C under moderate pressures.