Low-temperature phase transition and structural relationships of (CH3NH3)(3)Bi2Cl9

(CH3NH3)(3)Bi2Cl9 exhibits two phase transitions at T-c1 = 349 K and T-c2 2 47 K. The upper transition is probably isomorphous and reveals itself by th e 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 P2(1)2(1)2(1) as probable space group for the low-temperatu re phase with a = 20.43(1), b = 7.644(3), c = 13.225(4) Angstrom, and Z = 4 , Symmetry analysis allows one to ascribe nearly tricritical character to t he low-temperature phase transition. Structural relationships of (CH3NH3)(3 )Bi2Cl9 are displayed in the form of a group-subgroup diagram. (C) 2000 Aca demic Press.