Ordered K2NIF4 structure of the solids La2Li1/2M1/2O4 (M(III) = Co, Ni, Cu) and the bonding properties of the MO6 polyhedra in various compounds of this type

From single-crystal X-ray analyses it is derived that the compounds La2Li1/2M1/2O4 (M(III) = Co, Ni, Cu) crystallize in an orthorhombic variant of the K2NiF4 structure, with an ordered distribution of the LI(I) and M(III) cations on the octahedral sites. Accordingly, the unit cell is enlarged perpendicular to the [001] direction (space group Ammm, No. 65), though the metric remains tetragonal. The average M-O bond distances indicate low-spin configurations for the three 3d(n) cations, which is supported by spectroscopic measurements and ligand field considerations. The MO6 and LiO6 polyhedra are distinctly tetragonally elongated due to host site strains but superimposed by strong additional distortion components in the case of NiO6 and CuO6 as a consequence of vibronic Jahn-Teller coupling. Reported compounds of the same composition with M(III)= Al, Ga, and Mn seem to adopt the same structure. In contrast to the copper(III) compound, La2Li1/2Au1/2O4 crystallizes in the structurally related ordered Nd2CuO4-type lattice with square-planar AuO4 entities. (C) 1998 Academic Press.