Crystal chemistry of some novel rock salt- and perovskite-related oxides
The family of phases Li3M2XO6 (M = Mg, Co, Ni; X = Nb, Ta, Sb) has a novel rock salt superstructure in which the X cations occupy one set of octahedral sites with the Li and M cations distributed over three octahedral sites in a non-random manner. The non-random site occupancies vary depending on M and X and appear to be an equilibrium feature of the structure. Li3Ni2NbO6 undergoes a continuous order-disorder transition and can be doped with Cr3+ by a number of mechanisms to give either ordered or disordered rock salt structures. The phase diagram of the solid solutions of general formula Li3-xNi2-xCr xNbO6 has been determined and shows solid solutions for both ordered and disordered polymorphs with a transition zone that is 200-300A°C wide. Li3Ni2TaO6 and Cr-doped Li3Ni 2NbO6 compositions are modest semiconductors. The alpha and beta structures of polymorphic Li2CuZrO 4 have been determined. Both structures are ordered rock salt with unique sites for Zr and one set of Li atoms; in the alpha polymorph, Cu and the other set of Li atoms are partially ordered over two sites, but in beta-Li 2CuZrO4 all cations are uniquely ordered. In the phase transition from alpha-Li2CuZrO4 to the low temperature beta polymorph, the CuO6 octahedra, which predominantly corner-share in alpha, become edge-sharing. Magnetic susceptibility data indicate that, in both polymorphs, there are both ferromagnetic and antiferromagnetic interactions. The predominant interactions in the alpha phase are antiferromagnetic whereas in the beta polymorph, they are ferromagnetic. Conductivity measurements show that beta-Li2CuZrO4 is a modest semiconductor.