ORDERING OF TRANSITION-METAL ATOMS IN MCUFES(2) (M=LI, NA) - ELECTRONIC-STRUCTURE AND MONTE-CARLO SIMULATIONS

Previous X-ray investigations have shown the absence of long-range order in the distribution of transition metal atoms on the tetrahedral sites of the layered MCuFeS(2) (M = Li, Na) compounds. We have analyzed the band structure of these crystals for several distributions of Cu and Fe atoms by an extended Huckel Hamiltonian. The electronic structure study supports that Fe-Fe interactions are energetically more favorable than Cu-Fe contacts, Effective potential parameters for the interaction between transition metal atoms were derived from the calculated electronic energies. This potential model was used to investigate the equilibrium (Cu, Fe) distribution as a function of the temperature by Monte Carlo simulations. At the synthesis temperature of NaCuFeS2 (T-f = 1070 K), the (Cu, Fe) distribution does not display long-range order, a fact that agrees with X-ray diffraction studies, but is characterized by short-range order favoring the formation of chains and clusters of Fe atoms.