PHASE-TRANSITIONS IN (CH3NH3)4INBR7

Measurements of the dielectric constant, the DTA signal and the linear thermal expansion, optical observation of domain structure and X-ray examination of (CH3NH3)4InBr7 have been carried out over a temperature range from 25-degrees-C to 180-degrees-C. It is found that the crystal undergoes phase transitions at 171-degrees-C (T1) and 141-degrees-C (T2), and is ferroelastic below T1. The crystal at room temperature belongs to monoclinic P2/n with lattice parameters: a = 16.854(6) angstrom, b = 7.731(6) angstrom, c = 16.756(9) angstrom and beta = 103.22(2)-degrees. When the symmetry is approximated as to be pseudo-orthorhombic by taking a' = a + c, b' = b and c' = c - a (beta' = 90.34(11)-degrees), the domain structure observed below T1 is deduced from the strain-compatibility relations for the domain boundaries by taking account of the symmetry change from mmm to 2/m at T1. At T2 no change of domain configuration has been detected.