Low-Temperature Phase Transition and Structural Relationships of (CH3NH3)3Bi2Cl9

(CH3NH3)3Bi2Cl9 exhibits two phase transitions at Tc1=349 K and Tc2=247 K. The upper transition is probably isomorphous and reveals itself by the anomalous evolution of the lattice parameters. This behavior is explained to be due to increasing thermal motion of the methylammonium cations. The space group in both phases is Pnma. Single crystal X-ray work performed at 215 K indicates P212121 as probable space group for the low-temperature phase with a=20.43(1), b=7.644(3), c=13.225(4) ?, and Z=4. Symmetry analysis allows one to ascribe nearly tricritical character to the low-temperature phase transition. Structural relationships of (CH3NH3)3Bi2Cl9 are displayed in the form of a group–subgroup diagram.