The Ordered K2NiF4-type Structure of Mixed Crystals La2-xSrxLi1/2Co1/2O4 (x<0.5) and the Electronic Properties of the Constituting CoIII and CoIV Ions

Solids of the composition La2-xSrxLi1/2Co1/2O4 (x=0, x~=0.2) crystallize in a superstructure of the K2NiF4 lattice with a doubled unit cell in the (001) plane (space group Ammm; a=b= a0), caused by cation ordering between lithium and cobalt on the octahedral sites. The electronic structure of the CoIIIO6 and CoIVO6 polyhedra, which lie isolated in the lattice, were studied by various spectroscopic techniques (XANES, EPR, optical). The groundstates are low-spin in both cases (t62g and t52g, respectively), the ligand field parameters of CoIII being close to those characterizing the high-spin/low-spin crossover.