Hydrogen sorption in homologous lanthanum and cerium nickel silicides

The interaction of hydrogen with the La15Ni8Si9H31, Ce15Ni9Si8H35 (Pr15Ni7S i10 type structure) and CeNi0.6Si1.4 (AlB2-type structure) has been studied . These compounds belong to the homologous Rn2+3n+2T2n2+2 (R=La, Ce; T=Ni, Si, n=2, 3, 4, infinity) series with the Ce6Ni2Si3, Ce2NiSi, Pr15Ni7Si10 an d AlB2 structure types. The new hydride phases La15Ni8Si9H31 and Ce15Ni9Si8 H35 have been synthesised and characterised by X-ray diffraction and thermo desorption. CeNi0.6Si1.4 (n-->infinity, AlB2-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 un it cell distortion: the lattice parameter a and the unit cell volume upsilo n increased (da/a approximate to 0.07, dV/V approximate to 0.09), while the parameter c decreased (dc/c approximate to -0.04). The homologous Rn2+3n+2 T2n2+2 (R=La, Ce; T=Ni, Si) compounds form hydrides with general formula Rn 2+3n+2T2n2+2H-(2n2+3n+4) (n=2, 3, 4) with anisotropic unit cell distortion: the lattice parameter a and the unit cell volume u increased, while the pa rameter c decreased. Some suggestions on hydrogen localisation in the cryst al lattice and their anisotropic distortion have been proposed. (C) 2000 El sevier Science S.A. All rights reserved.