Thermodynamics of copper sulfides I. Heat capacity and thermodynamic properties of copper(I) sulfide, Cu2S, from 5 to 950 K

The heat capacity of Cu2S from 5 to 950 K was determined by adiabatic-shield calorimetry and the thermodynamic properties were evaluated. Transitions occur at 376 and about 710 K with [Delta]trsSm = (1.240+/-0.006)R and (0.201+/-0.006)R. Considerable hysteresis was involved in achieving equilibrium in the latter transition. At 298.15 and 950 K the values of Cp,m(T), Smo(T), and [Phi]mo(T, 0) are 9.242R, 13.987R, and 7.6121R; and 9.960R, 27.810R, and 17.875R, respectively. Subtraction of the estimated lattice heat capacity at constant pressure leaves a large excess heat capacity, especially for the fast ionic conductor [beta]-Cu2S. It is about 2.8R at 400 K and decreases gradually to 1.26R at 680 K. Its origin is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27001/1/0000568.pd