From the Laves Phase CaRh to the Perovskite CaRhH-in Situ Investigation of Hydrogenation Intermediates CaRhH

The hydrogenation properties of the cubic Laves phase CaRh and the formation of the perovskite CaRhH were studied by in situ thermal analysis (differential scanning calorimetry), sorption experiments, and in situ neutron powder diffraction. Three Laves phase hydrides are formed successively at room temperature and hydrogen gas pressures up to 5 MPa. Cubic α-CaRhH is a stuffed cubic Laves phase with statistically distributed hydrogen atoms in tetrahedral [CaRh] voids (ZrCrH type, Fd3̅ m, a = 7.5308(12) Å). Orthorhombic β-CaRhD (own structure type, Pnma, a = 6.0028(3) Å, b = 5.6065(3) Å, c = 8.1589(5) Å) and γ-CaRhD (β-CaRhH type, Pnma, a = 5.9601(10) Å, b = 5.4912(2) Å, c = 8.0730(11) Å) are low-symmetry variants thereof with hydrogen occupying distorted tetrahedral [CaRh] and trigonal bipyramidal [CaRh] voids. Hydrogen sorption experiments show the hydrogenation to take place already at 0.1 MPa and to yield β-CaRhH. At 560 K and 5 MPa hydrogen pressure the Laves phase hydride decomposes kinetically controlled to nanocrystalline rhodium and CaRhD (CaTiO type, Pm3̅ m, a = 3.6512(2) Å). The hydrogenation of CaRh provides a synthesis route to otherwise not accessible perovskite-type CaRhH.