P-T PHASE-RELATIONS OF CH3NH3PBX3 (X=CL, BR, I) CRYSTALS

Pressure-temperature phase relations of CH3NH3PbX3 (X = Cl, Br, I) crystals were studied by using a high pressure DTA apparatus in the range between 0.1 Pa and 200 MPa. A triple point was found in each compound below 100 MPa. By pressurization, the low pressure phases disappeared at the triple point in the chloride and bromide while a new high-pressure phase appeared in the iodide. The pressures and temperatures of the triple points are 75.1 MPa, 175.7 K for CH3NH3PbCl3, 43.2 MPa, 152.9 K for CH3NH3PbBr3, and 84.8 MPa, 176.2 K for CH3NH3PbI3. All the boundaries between the cubic and tetragonal phases are upward convex and that of the iodide has a maximum at about 120 MPa. Other phase boundaries are essentially straight lines in the measured pressure and temperature ranges. By the use of the Clausius-Clapeyron equation, the transition volumes were calculated from the slopes of the phase boundaries and the transition entropies obtained in a previously published calorimetric experiment (J. Phys. Chem. Solids 51, 1383 (1990)).