From the Laves Phase CaRh2 to the Perovskite CaRhH3–in Situ Investigation of Hydrogenation Intermediates CaRh2Hx

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-CaRh2H0.05 is a stuffed cubic Laves phase with statistically distributed hydrogen atoms in tetrahedral [Ca2Rh2] voids (ZrCr2H3.08 type, Fd3m, a = 7.5308(12) A). Orthorhombic β-CaRh2D3.93(5) (own structure type, Pnma, a = 6.0028(3) A, b = 5.6065(3) A, c = 8.1589(5) A) and γ-CaRh2D3.20(10) (β-CaRh2H3.9 type, Pnma, a = 5.9601(10) A, b = 5.4912(2) A, c = 8.0730(11) A) 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-CaRh2H3.8(2). At 560 K and 5 MPa hydrogen pressure the Lave...