Phase Transitions in (CH3NH3)4lnBr7

Measurements of the dielectric constant, the DTA signal and the linear thermal expansion, optical observation of domain structure and X-ray examination of (CH3NH3)4lnBr7 have been carried out over a temperature range from 25°C to 180°C. It is found that the crystal undergoes phase transitions at 171°C (T1) and 141 °C (T2), and is ferroelastic below T1. The crystal at room temperature belongs to monoclinic P2/n with lattice parameters: a= 16.854(6) Å, b = 7.731(6) Å, c= 16.756(9) Å and 103.22(2)°. When the symmetry is approximated as to be pseudo-orthorhombic by taking a'=a + c, b’ = b and c’ = c-a (β =90.34(11)°), 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.