The dielectric study of Sr 0.73 Ba 0.27 Nb 2 O 6 :Ce (SBN) crystals along [010] crystallographic axis was performed in the temperature region of 310-360 K and frequency range from 25 Hz up to 1 MHz. The thermal dipole relaxation of quasi-Debye-type for this orientation of sample was observed in both investigated structural phases. The phase transition was most clearly seen from the temperature dependence of the relaxation time at T c = 320 K. The relaxation processes were related to the collective oscillations of the Nb-O bonds in the two different type corner-sharing NbO 6 octahedra aligned along c-axis.
We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH 3 ) 4 ) 2 Zn 0.8 Ni 0.2 Cl 4 crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity.Introduction. The scientific interest in tetramethylammonium tetrachlorozincate crystals (N(CH 3 ) 4 ) 2 ZnCl 4 is connected with the fact that they can be considered as model systems with incommensurable modulation. It is specifically the incommensurable modulation that is associated with such specific phenomena as global thermal hysteresis, special cycles of the "parallelogram" and "dielectric memory" type, and also the thermal memory effect which, in particular, can be observed as the thermooptic memory (TOM) effect in measurement of optical parameters [1,2]. The memory effect can be associated with localization of the incommensurable modulation vector at a higher order commensurable value [3,4].TMAZC crystals belong to the family [N(CH 3 ) 4 ] 2 XB 4 (X = Zn, Co, Cu, Mn, Fe, Cd, Ni; B = Cl, Br). Its characteristic feature is the presence of a complex series of ferroelectric, ferroelastic, and incommensurable phases [5]. The (N(CH 3 ) 4 ) 2 ZnCl 4 crystal is no exception, undergoing five phase transitions:
The temperature evolution of absorption spectra of isopropylammonium trichlorocuprate (C 3 H 7 NH 3 CuCl 3 ) crystals was investigated in the energy range 1.77-2.48 eV. It was established that the optical absorption edge was described by the Urbach rule and resulted from exciton-phonon interaction. A comparatively strong exciton-phonon interaction was associated with a band produced by self-trapped excitons. Analysis of the temperature dependences for the Urbach-rule parameters confirmed the existence of a thermochromic phase transition at 325 K.
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