IMF2017

In some hybrid crystals with a perovskite-like structure containing propylenediammonium cations (NH₃(CH₂)₃NH₃) and layered metal-halogen anionic sublattices, a “inverted” sequence of the symmetry change was observed. The change of symmetry from higher to lower one was detected during heating. Such reduction of the structure symmetry from orthorhombic to monoclinic was found for e.g. in (NH₃(CH₂)₃NH₃CdBr₄ and [NH₃(CH₂)₃NH₃]CuCl₄ crystals. The aim of the work is to give a more detailed description of the domain structure changes during the “inverted” transitions. It was shown that the orthorhombic mmm phase should be regarded as the prototype phase for the ferroelastic transition to the monoclinic phase. A critical index for the order parameter was determined from the temperature dependence of the linear birefringence at the continuous transition between orthorhombic phases.