SYNTHESIS AND STRUCTURES OF THE PARTIALLY ORDERED ROCK-SALT PHASES, LI(3)M(2)XO(6) - M=MG, CO, NI, X=NB, TA, SB

The crystal structures of four rock salt superstructure phases, Li(3)M(2)XO(6)(M(2)X=Ni2Nb, Ni2Ta, Co2Ta and Mg2Nb) have been refined by X-ray and/or neutron powder diffraction; three members of this family were synthesized for the first time, M(2)X=Ni2Nb, Ni2Sb and Co2Ta. All have similar rock salt superstructures but with partial order only, over three sets of octahedral sites, of the cations M(2)X. Differences in the distributions of site occupancies are discussed.