Silver(I) sulfide: Ag 2S Heat capacity from 5 to 1000 K, thermodynamic properties, and transitions

Abstract

The heat capacity of Ag 2S 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 α-to-β transition of Ag 2S occurs in the range 449.3 to 451.3 K, depending upon previous history of the sample. The enthalpy of transition Δ trs H m = (4058±26) J·mol −1. A slightly decreasing heat capacity is observed for β-Ag 2S from 88.1 J·K −1·mol −1 at 460 K to 85.0 J·K −1·mol −1 at 850 K with a minimum of 84.6 J·K −1·mol −1 at 750 K. The transition of β-Ag 2S to γ-Ag 2S occurs at about 865 K with Δ trs H m = (784±5) J·mol −1. Thermodynamic functions have been evaluated and the values of C p,m , “S m o (T)−S m o (0)’ , and −“G m o (T)−H m o (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·K −1·mol −1, respectively. No signs of further transitions were found, either in the stoichiometric compound, or in a sample with overall composition Ag 2S 1.0526. Thus, the present work does not support the hypothesis of Perrott and Fletcher concerning partial disordering of stoichiometric Ag 2S 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 β-Ag 2S above 450 K. It is about 11 J·K −1·mol −1 at 500 K and decreases gradually to about 6 J·K −1·mol −1 at 850 K. Its origin is discussed.