A new hexagonal Laves phase deuteride CeMn1.5Al0.5Dx (0<x<4) -: Investigated by in situ neutron diffraction

The crystal structure of the C14 (P6(3)/mmc) hexagonal Laves phase compound CeMn1.5Al0.5Dx (x<4) was investigated using in situ neutron powder diffraction during deuteriding. First, real-time neutron scattering measurements were performed during the deuteriding of the compound from 0 to 170 ncm(3) D-2/g-sample (3.9 D/fu). From these measurements we determined the critical concentrations at which phase transitions occurred. In this compound, the deuterium initially dissolved into the host-metal lattice forming a solid solution (alpha-phase). This phase exhibited an isotropic lattice expansion with increasing deuterium concentration. The nucleation of a deuteride (the beta phase) occurred at a total deuterium content of 20 ncm(3) D-2/g-sample. The transformation into the deuteride was complete at 145 ncm(3) D-2/g-sample, giving a stoichiometry of CeMn1.5Al0.5D3.0 for the beta phase. With increasing pressure, this phase continued to absorb deuterium forming a beta' deuteride solid solution. The growth of the beta phase is accompanied by the occurrence of a broad peak in the background. The position and diffuse character of this peak clearly indicates the presence of short-range deuterium ordering in the otherwise disordered occupation of interstitial sites. Finally, the crystal structures of all CeMn1.5Al0.5Dx (x<4) phases were precisely determined using high resolution neutron powder diffraction. The host metal compound, CeMn1.5Al0.5, is a pseudo-binary AB(2)-type compound showing a statistical distribution of Mn and Al on two different B sites. In the CeMn1.5Al0.5Dx phases, deuterium was located only on A2B2-type tetrahedral interstitial sites. The occupation of these specific interstitial sites can be explained in terms of diffusion paths, maximized D-D distances, as well as, the preference of deuterium for interstitial sites coordinated by the largest number of Ce second-nearest-neighbors atoms. (C) 2000 Elsevier Science S.A. All rights reserved.