Thermodynamic properties of NiAs-Type Co1±xSb and Ni1±xSb

A statistical model is developed that describes the composition dependence of the thermodynamic activities in phases with the NiAs-(B8-) structure. The model is based on vacancies in the transition metal sublattice (octahedral sites) and additional transition metal atoms in interstitial positions (trigonal-bipyramidal sites) as intrinsic disorder. It can be applied to both types of defect mechanisms, i.e., NiAs-type (where vacancies are assumed to be distributed statistically over all transition metal sites) and CdI2-type (where the vacancies are restricted to alternate layers of the transition metal sublattice); any kind of intermediate behavior with different vacancy concentrations on alternate transition metal layers can also be described. The model is applied to the two phases Co1±xSb and Ni1±xSb for which antimony activities were determined by an isopiestic vapor pressure method.