From the Laves Phase CaRh2 to the Perovskite CaRhH3-in Situ Investigation of Hydrogenation Intermediates CaRh2H x

Abstract

The hydrogenation properties of the cubic Laves phase CaRh₂ and the formation of the perovskite CaRhH₃ 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-CaRh₂H_0.05 is a stuffed cubic Laves phase with statistically distributed hydrogen atoms in tetrahedral [Ca₂Rh₂] voids (ZrCr₂H_3.08 type, Fd3̅ m, a = 7.5308(12) Å). Orthorhombic β-CaRh₂D_3.93(5) (own structure type, Pnma, a = 6.0028(3) Å, b = 5.6065(3) Å, c = 8.1589(5) Å) and γ-CaRh₂D_3.20(10) (β-CaRh₂H_3.9 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 [Ca₂Rh₂] and trigonal bipyramidal [Ca₃Rh₂] voids. Hydrogen sorption experiments show the hydrogenation to take place already at 0.1 MPa and to yield β-CaRh₂H_3.8(2). At 560 K and 5 MPa hydrogen pressure the Laves phase hydride decomposes kinetically controlled to nanocrystalline rhodium and CaRhD_2.93(2) (CaTiO₃ type, Pm3̅ m, a = 3.6512(2) Å). The hydrogenation of CaRh₂ provides a synthesis route to otherwise not accessible perovskite-type CaRhH₃.