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 characteri zed by a room-temperature x-ray diffraction method to be of single pha ses 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 wer e measured by adiabatic calorimetry in the temperature range between 1 3 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 sk irts 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), re spectively, 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 th e phase transition are discussed.