Abstract
The heat capacities of Co 3Se 4 and Co 3Te 4 have been determined by adiabatic shield calorimetry in the range 300 to 1000 K. Values of heat capacity, enthalpy, and entropy, obtained from least-squares treatment and integration of the results are tabulated for selected temperatures. In Co 3Se 4 a λ-type transition occurs, with a heat-capacity maximum of 143.7 J K −1 for ( 1 7 ) Co 3Se 4 over a 1.5 K temperature interval around 1005 K, presumably connected with disordering of vacancies in the grossly defective NiAs-like structure. The estimated order-disorder entropy increment is about 2.6 J K −1 mol −1 for Co 0.75Se, which is slightly smaller than for the assumed disordering of 1 4 mol of Co and vacancies ( ΔS = 2.88 J K −1 mol −1). For Co 3Te 4 normally increasing heat capacities are observed. The cobalt atoms and vacancies in the partially filled metal layers thus probably remain disordered down to the lowest temperatures. The configurational zero-point entropy of the telluride is therefore presumably about 1.7 J K −1 mol −1 for ( 1 7 ) Co 3Te 4 . An analysis of vacancy ordering and phase stability in structure of NiAs-like type has been carried out in terms of vacancy formation and vacancy pair formation energies. The influence of structural transitions on the phase relations and on the occurrence of stable disordered regions is demonstrated.
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