Abstract:Crystal structure of annealed alloy La0.67Mg0.33Ni2.5Co0.5 and (La0.67Mg0.33Ni2.5Co0.5)D4.0 deuterides were studied by X-ray diffraction, neutron diffraction and Rietveld refinement. It is found that the alloys are mainly composed of (La,Mg)Ni3 phase with rhombohedra PuNi3 type (space group: R3m) structure, and the second phases are LaMgNi4 phase (MgCu4Sn type) and La2Ni7 phase (Ce2Ni7 type). In the (La,Mg)Ni3 phase, the Mg atoms only occupy at 6c sites in the RM2 unit, while the Co atoms mainly occupy at 18 h in the RM5 unit and 6c positions in the boundary between the RM5 and RM2 blocks. The full deuteride of the main phase (La0.67Mg0.33Ni2.5Co0.5)D4.0 has a rhombohedral structure. For the (La0.67Mg0.33Ni2.5Co0.5)D4.0 deuteride, the significant occupancy of deuterium atoms are mainly six sites: 6c1, 18h3 and 18h1 within the RM2 units and 36i2, 18h5, 6c4 within the RM5 units. At the same time, it is found that the 6.5(1)D/f.u. is located within the RM5 units whereas the 3.2(2) )D/f.u. belongs to the RM2 units. For the PuNi3 type structure of (La0.67Mg0.33Ni2.5Co0.5)D4.0, the total isotropic lattice expansions of the a-axis and c-axis are δa/a＝7.1% and δc/c=9.1%, respectively. A significant anisotropic volume expansion (δc/c=15.8%, δV/VRM2=32.8%) is found inside the RM2 unit, but inside the RM5 unit, the volumetric expansions are 2.4% (δc/c) and 17.5% (δV/VRM5)