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 La 2Li1/2M1/2O4 (M(III) = Co, Ni, Cu) crystallize in an orthorhombic vari ant of the K2NiF4 structure, with an ordered distribution of the LI(I) and M(III) cations on the octahedral sites. Accordingly, the unit cel l is enlarged perpendicular to the [001] direction (space group Ammm, No. 65), though the metric remains tetragonal. The average M-O bond di stances indicate low-spin configurations for the three 3d(n) cations, which is supported by spectroscopic measurements and ligand field cons iderations. The MO6 and LiO6 polyhedra are distinctly tetragonally elo ngated 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 composi tion 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-plan ar AuO4 entities. (C) 1998 Academic Press.