Silver(I) sulfide: Ag2S Heat capacity from 5 to 1000 K, thermodynamic properties, and transitions

Abstract

The heat capacity of Ag2S has been measured by adiabatic-shield calorimetry from 5 to 1000 K. The heat capacity increases regularly up to about 445 K where the pre-transitional contribution causes rapidly rising values. The [alpha]-to-[beta] transition of Ag2S occurs in the range 449.3 to 451.3 K, depending upon previous history of the sample. The enthalpy of transition [Delta]trsHm = (4058+/-26) J[middle dot]mol-1. A slightly decreasing heat capacity is observed for [beta]-Ag2S from 88.1 J[middle dot]K-1[middle dot]mol-1 at 460 K to 85.0 J[middle dot]K-1[middle dot]mol-1 at 850 K with a minimum of 84.6 J[middle dot]K-1[middle dot]mol-1 at 750 K. The transition of [beta]-Ag2S to [gamma]-Ag2S occurs at about 865 K with [Delta]trsHm = (784+/-5) J[middle dot]mol-1. Thermodynamic functions have been evaluated and the values of Cp,m, "Smo(T)-Smo(0)', and -"Gmo(T)-Hmo(0)'/T at 298.15 K are 75.31, 142.89, 85.43, and at 1000 K are 80.57, 253.28, 172.77 J[middle dot]K-1[middle dot]mol-1, respectively. No signs of further transitions were found, either in the stoichiometric compound, or in a sample with overall composition Ag2S1.0526. Thus, the present work does not support the hypothesis of Perrott and Fletcher concerning partial disordering of stoichiometric Ag2S around 600 K as opposed to complete disordering around 450 K in the presence of excess silver or sulfur. Subtraction of the estimated lattice heat capacity at constant pressure leaves a large transitional heat capacity for [beta]-Ag2S above 450 K. It is about 11 J[middle dot]K-1[middle dot]mol-1 at 500 K and decreases gradually to about 6 J[middle dot]K-1[middle dot]mol-1 at 850 K. Its origin is discussed.