Changes in the optical properties at phase transitions in TEA2MeCl4 (Me=Zn, Mn, Hg, Cu) crystals

Optical properties of TEA2MeCl4 crystals, where Me is a divalent metal ion, have been studied under polarization microscope in temperatures ranging from 200 to 300 K. Abrupt changes in the interference colours and strong cracking of the sample have been observed for all crystals except the TEA2CuCl4 one, for which only brightening of the plate image has been noted. The cracks are approximately parallel to the cleavage planes. These changes occur in two stages. The brightening of the images of the crystal samples may suggest the ferroelastic deformation in microregions. No classical ferroelastic domain structure has been observed in the low-temperature phase.