A Powder Neutron Diffraction Study of the LaNi₄.₅Al0.₅D₄.₅ Structure at 298 and 77 °K

Recently, there has been some controversy over the structure of hydrides of the La(Ni//5// minus //xAl//x)//5 system. Once of the proposed structures belongs to the P6/mmm space group and contains five deuterium sites. The other proposed structure belongs to the P31 m space group and contains two deuterium sites. In an attempt to resolve this conflict, a powder neutron diffraction investigation has been carried out for LaNi//4//. //5Al//0//. //5D//4//. //5 at 298 and 77 degree K. A modified Rietveld profile analysis technique was used to interpret the data. It is found that a two-site model using P6/mmm symmetry adequately describes the structure at 298 degree K. At 77 degree K, there is no evidence of magnetic ordering and the structure shows only slight changes from that given at 298 degree K. The lattice parameter a contracts from 5. 341 to 5. 331 A, while c remains constant at 4. 236 A