A new hexagonal Laves phase deuteride CeMn 1.5Al 0.5D [formula omitted] (0< [formula omitted]

Abstract

The crystal structure of the C14 ( P6 3/mmc ) hexagonal Laves phase compound CeMn 1.5Al 0.5D x ( 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 (α-phase). This phase exhibited an isotropic lattice expansion with increasing deuterium concentration. The nucleation of a deuteride (the β 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 CeMn 1.5Al 0.5D 3.0 for the β phase. With increasing pressure, this phase continued to absorb deuterium forming a β′ deuteride solid solution. The growth of the β 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 CeMn 1.5Al 0.5D x ( x<4) phases were precisely determined using high resolution neutron powder diffraction. The host metal compound, CeMn 1.5Al 0.5, is a pseudo-binary AB 2-type compound showing a statistical distribution of Mn and Al on two different B sites. In the CeMn 1.5Al 0.5D x 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.