Hydrogen sorption in homologous lanthanum and cerium nickel silicides

Abstract

The interaction of hydrogen with the La 15Ni 8Si 9H 31, Ce 15Ni 9Si 8H 35 (Pr 15Ni 7Si 10 type structure) and CeNi 0.6Si 1.4 (AlB 2-type structure) has been studied. These compounds belong to the homologous R n 2+3 n+2 T 2 n 2+2 (R=La, Ce; T=Ni, Si, n=2, 3, 4, ∞) series with the Ce 6Ni 2Si 3, Ce 2NiSi, Pr 15Ni 7Si 10 and AlB 2 structure types. The new hydride phases La 15Ni 8Si 9H 31 and Ce 15Ni 9Si 8H 35 have been synthesised and characterised by X-ray diffraction and thermodesorption. CeNi 0.6Si 1.4 ( n→∞, AlB 2-type structure) did not react with hydrogen at a pressure up to 200 MPa at room temperature. The hydrides preserved the structure type of the starting compounds with anisotropic unit cell distortion: the lattice parameter a and the unit cell volume v increased (d a/ a≈0.07, d V/ V≈0.09), while the parameter c decreased (d c/ c≈−0.04). The homologous R n 2+3 n+2 T 2 n 2+2 (R=La, Ce; T=Ni, Si) compounds form hydrides with general formula R n 2+3 n+2 T 2 n 2+2 H ∼(2 n 2+3 n+4) ( n =2, 3, 4) with anisotropic unit cell distortion: the lattice parameter a and the unit cell volume v increased, while the parameter c decreased. Some suggestions on hydrogen localisation in the crystal lattice and their anisotropic distortion have been proposed.