SPECIFIC-HEAT OF BETA-AGI SINGLE-CRYSTAL AT LOW-TEMPERATURES

Using high resolution AC calorimetry between 10 and 300 K, we measured the specific heat of AgI single crystal grown at room temperature in the hexagonal (P) phase. In addition to the previously reported therma l anomaly at T-1 similar to 60 K, a second peak in the specific heat w as observed at T-2 similar to 220 K. Although the thermal signal at T- 1 was reversible and shows no dependence on the thermal history of the sample, the peak at T-2 was enhanced on successive temperature scanni ng of the sample through the transition point until it reached a const ant area (enthalpy) of about 4.15 J mol(-1). We discuss our observatio ns near 220 K in terms of a phase transition in the silver ion sublatt ice in beta-AgI due to disordering of the mobile ions on heating the s ample through the transition temperature. (C) 1998 Elsevier Science B. V. All rights reserved.