The dielectric parameters and electric conductivity of (NH2(CH3)2)2CoCl4 crystals are investigated. Existence of the ferroelectric phase was observed below Tc = 258 K. The occurrence of phase transitions at 244, 314, and 435 K is confirmed. The investigated crystals also manifest anomalously high conductivity commensurate with that in semiconductors. Its value as well as region of existence strongly depend on the preceding history of the sample. It is shown that the anomalous conductivity is connected with translocation of protons which participate in the formation of hydrogen bonds.
The temperature evolution of the optical absorption spectra of A2BX4 type compounds with an organic cation have been analyzed. It was shown that the fundamental absorption edge in these materials possesses an exponential shape following the empirical Urbach rule due to the occurrence of the electron–(exciton–)phonon interaction. As an example, the parameters of the EPI have been determined for (NH2(CH3)2)2CuCl4 crystals. Comparison of the obtained results with corresponding data for isomorphous compounds allowed to ascertain some total regularities characteristic of the EPI. Nonfulfilment of the Urbach rule in the incommensurate phase, particularly, was related to the considerable temperature dependence of the period and phase of the modulation. This conclusion is confirmed by data of Raman spectroscopy.
On the basis of dilatometric, optical, and dielectric investigations of (NH3C2H5)2CuC14 crystals the existence of phase transitions at T1 = 364 and T2 = 356 K was confirmed. Anomalous behaviour of the thermal expansion coefficient and optical birefringence around T3 = 330 K was related to earlier unknown phase transition. Besides, it was shown that these crystals are characterised by considerable protonic conductivity, the nature of which is fairly well explained in the framework of Grotthus mechanism of proton transport.
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