Birefringence of low-temperature phases of ferroelastic bis(n-propylammonium) tetrachlorocadmate

Polarized light observations accompanied by precise birefringence measurements (within the (001) plane) of bis(n-propylammonium) tetrachlorocadmate, (n-C3H7NH3)2CdCl4, were performed at 8-300 K. Existence of 1st order phase transitions (at 105(1) K and 157(1) K) and of a 2nd order incommensurate phase transition (at 180 K) were clearly confirmed. All 4 low-temp. phases are ferroelastic. Clear manifestation of the spontaneous shear angle was noticed; its large room-temp. value detd. optically agrees with x-ray diffraction results. Good correlation of birefringence behavior with adiabatic calorimetry and Raman scattering results was found