Structure and Properties of Hydrogenated Ca, Sr, Ba, and Eu Silicides

Abstract

AbstractThe binary metal silicides CaSi, BaSi, SrSi, and EuSi that crystallize in the CrB type absorb reversibly hydrogen. Hydrogen contents of the products were measured by the carrier gas hot extraction method (CaSiHx: 1.56 wt %, x ≈︁ 1.06; SrSiHx: 1.35 wt %, x ≈︁ 1.6; BaSiHx: 2.08 wt %, x ≈︁ 3.4; EuSiHx: 0.99 wt %, x ≈︁ 1.8). According to simultaneous thermogravimetric, DTA and mass‐spectroscopic analyses, the dehydrogenation for the different hydrides starts at 610 K for BaSiHx, at 675 K for CaSiHx and SrSiHx, and at 640 K for EuSiHx. BaSiHx releases further hydrogen at 765 K and 1000 K, respectively, CaSiHx at 760 K. Fundamental morphological and structural changes caused by the hydrogenation of the silicides were detected and comprehensively characterized by SEM and X‐ray diffraction. According to XRD results, the crystal structures of the phases MSiHx (M = Ca, Ba, Sr, Eu) are all orthorhombic (space group Pnma). The lattice parameters change from a = 4.559(1) Å, b = 10.731(1) Å, c = 3.890(1) Å (space group Cmcm, Z = 1) in CaSi to a = 14.4884(9) Å, b = 3.8247(1) Å, c = 11.2509(5) Å (Z = 3) in CaSiHx caused by the hydrogen uptake that induces the formation of a threefold superstructure along the a axis. In CaSiHx, the planes of the [Sin]2– zigzag chains are tilted and inter‐chain Si–Si contacts are formed formally corresponding to an oxidation to Si–. The hydrogen desorption process was investigated by in situ X‐ray diffraction studies which revealed the presence of two different hydrogen species in CaSiHx. The new phase SrSiHx crystallizes in a related structure comprising tilted [Sin]2– chains (a = 15.1051(5) Å, b = 3.95400(8) Å, c = 12.3431(3) Å, space group Pnma). The lattice parameters of BaSiHx are a = 15.7(2) Å, b = 4.13(3) Å, c = 13.43(8); that of EuSiHx a = 15.17 Å, b = 3.95 Å, c = 11.56 Å (Z = 3).