Fractional Site Occupation in Ternary Metal Compounds: Structure, Bonding, and Thermodynamics

Summary.

Ternary compounds of the type (M,M′) _x A _y where M and M′ are early transition metals of the groups 4–6 and A is a main group element of the groups 14–16 are showing interesting substitution mechanisms among the metal atoms ranging from classical and partially ordered solid solution phases to ternary compounds showing differential fractional site occupation. In these compounds the transition metals show mixed site occupation at the metal positions in combination with pronounced site preferences leading to varying metal mixtures at crystallographically independent sites. The connection between partial ordering and the differences in the local coordination of the respective lattice sites is discussed. Chemical bonding arguments obtained from electronic calculations using the extended Hückel approach are used to understand the observed distribution of the metals over the respective lattice sites and allow a qualitative prediction of site preferences. A thermodynamic model was applied in order to investigate the observed substitution mechanism and Gibbs energies for the occupation of the lattice sites with different metal atoms could be obtained by adjusting the model parameters to the experimentally observed site fractions.