Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH3)4)2Zn0.8Ni0.2Cl4 solid solutions

Abstract

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.