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 (CH 3 NH 3 ) 4 InBr 7 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 ( T 1 ) and 141°C ( T 2 ), and is ferroelastic below T 1 . 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 β=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 T 1 is deduced from the strain-compatibility relations for the domain boundaries by taking account of the symmetry change from mmm to 2/ m at T 1 . At T 2 no change of domain configuration has been detected.