A CALORIMETRIC STUDY ON THE ORDER-DISORDER TRANSITION OF SILVER IONS IN AG0361TIS2 - THE 2-DIMENSIONAL NATURE OF THE TRANSITION

AgxTiS2 intercalation compounds were thermally prepared and characterized by a room-temperature x-ray diffraction method to be of single phases in stage 1, stage 2 and stage 1' in the ranges x = 0.36-0.42, 0.17-0.22 and 0-0.12, respectively. The heat capacities of Ag0.361TiS2 were measured by adiabatic calorimetry in the temperature range between 13 and 400 K. The intralayer order-disorder phase transition of silver ions was observed at 285.5+/-0.3 K as a higher-order type with wide skirts of anomalous heat capacity on the low- and high-temperature sides of the peak. The enthalpy and entropy of the transition were found to be 4.3+/-0.4 kJ mol-1 and 9.9+/-1.0 J K-1 mol-1 (close to R ln 3), respectively, on the assumption of a high-temperature approximation. The interaction of positional ordering of silver ions is suggested to be of a two-dimensional nature, and two theoretical models relevant to the phase transition are discussed.