Heat capacities and thermodynamic properties of iron diselenide and nickel diselenide from 300 to 1000 K

Abstract

The heat capacities of iron diselenide and nickel diselenide have been determined in the range 300 to 1000 K by adiabatic shield calorimetry. The heat capacity of iron diselenide shows a remarkable increase from about 71 J K −1 mol −1 at 300 K to 200 J K −1 mol −1 at 856.7 K, where a peritectic decomposition into the solid Fe 1–2Se phase with approximate composition Fe 3Se 4, and a selenium-rich liquid takes place. The heat capacity increase is attributed mainly to defect formation in the marcasite-type structure, which overshadows any contributions from compositional changes of the diselenide phase and excitable electronic states (low to high spin). The enthalpy of decomposition at 856.7 K is (30.93 ± 0.05) kJ mol −1 for the stoichiometric sample. In the solid + liquid region a heat capacity maximum of about 200 J K −1 mol −1 is observed around 1000 K due to the structural order-disorder transformation in the Fe 1–2Se phase. For nickel diselenide a normally increasing heat capacity is encountered, reaching about 92 J K −1 mol −1 at 1000 K. Values of thermodynamic functions have been calculated and are tabulated for selected temperatures.