Heat capacities and thermodynamic properties of Cr 3Te 4 and Ni 3Te 4 from 298 to 950 K. Structural order-disorder transitions

Abstract

The heat capacities of Cr3Te4 and Ni3Te4 have been determined in the range 298 to about 950 K by adiabatic-shield calorimetry with intermittent electrical energy inputs and temperature equilibration between inputs. Values of heat capacity, enthalpy, and entropy are tabulated for selected temperatures after inclusion of earlier low-temperature results. For Cr3Te4 a λ-type transition occurs with heat capacity maximum at 903 K, presumably connected with disordering of vacancies in the grossly defective structure. The estimated order-disorder entropy change is 2.2 J K−1 mol−1 for Cr0.75Te, which compares reasonably well with the assumed disordering of 1/4 mol of Cr and vacancies (ΔS = 2.88 J K−1 mol−1). For Ni3Te4 no transition occurs, except for a slightly enhanced heat capacity in the region 350 to 500 K, indicating only a slight tendency for vacancy ordering under the conditions in question. The vacancies thus probably remain largely disordered in every other metal layer parallel to 001 down to the lowest temperatures, and the zero-point configurational entropy is accordingly approximately 1.5 J K−1 mol−1 for 1/7 Ni3Te4.