Structural phase transition in single-crystal CH₃NH₃CdBr₃: I. Experimental studies

Single crystals of CH₃NH₃CdBr₃ (MACB) have been studied as a function of temperature by means of differential scanning calorimetry, X-ray diffraction and Raman scattering measurements. A first-order structural phase transition is evidenced by all three techniques at 170 K. The high-temperature phase (I) is hexagonal, with space group P6₃/mmc and Z=2 formula units per unit cell. The X-ray diffraction patterns of the low-temperature phase (II) are consistent with a complex orthorhombic unit cell with lattice parameters such as a_II approximately=4a_hex, b_II approximately=2 square root 3a_hex and c_II approximately=3c_hex, i.e. corresponding to Z=96; however, it is shown that the system can be described in a simplified manner by means of an orthorhombic 'projection' unit cell onto the hexagonal plane with parameters such as a_p approximately=a_hex, b_p approximately= square root 3a_hex and c_p approximately=3c_hex. The Raman data clearly indicate that the phase transition is of the order-disorder type, owing to the reorientational processes of the methylammonium groups.