CALORIMETRIC AND IR SPECTROSCOPIC STUDIES OF PHASE-TRANSITIONS IN METHYLAMMONIUM TRIHALOGENOPLUMBATES-(II)

Heat capacities of CH3NH3PbX3 (X = Cl, Br, I) were measured between 13 and 300 K (365 K for the iodide). Two anomalies were found in the chloride and the iodide, and three in the bromide. All the phase transitions were of the first order, although the highest temperature transitions in the bromide and the iodide were close to second order. Their temperatures and entropies are as follows: CH3NH3PbCl3: 171.5 K (14.6 J K-1mol-1), 177.2 K (10.0 J K-1mol-1); CH3NH3PbBr3: 148.8 K (11.2 J K-1mol-1), 154.0 K (4.1 J K-1mol-1), 236.3 K (8.2 J K-1mol-1); CH3NH3PbI3: 161.4 K (19.0 J K-1mol-1), 330.4 K (9.7 J K-1mol-1). The transition entropies indicated that the phase transitions are of the order-disorder type. They are interpreted with three possible models involving the methylammonium ions disordered with respect to the orientation of the C-N axis itself and around the C-N axis. The infrared line width of the v12 vibration of the methylammonium ion depended markedly on the temperature and was interpreted as caused by the hindered rotational motion in the cubic and tetragonal phases.