The temperature dependences of optical birefringence δ(Δn) for the principal cuts of [N(CH_{3})_{4}]_{2}CuCl_{4} crystals are measured. On the basis of the obtained experimental and theoretical data the anomalous behavior of physical parameters in conditions of the "viscous" interaction of the modulated structure with mobile defects in the incommensurate phase is explained.
The temperature dependences of the optical birefringenceδ(Δ n_c) in (MgGeF_6)·6H_2O crystal have been investigated. Specific properties observed in experiment such as global hysteresis, kinetic effect, thermooptic memory effect, localization of the incommensurability wave vector on the commensurate values of higher order, sensibility to the external mechanical stress confirm the existence of the incommensurate phase in current crystals.
Temperature behavior of the optical birefringence in the transitional regions of the incommensurate phase of [N(CH_3)]CuCl_4 was studied. Temperature dependencies of the modulated structure wave vector and of the specific heat in these regions were analyzed. The stochastic mode of the incommensurate modulated structure in [N(CH_3)]MeCl_4 (Me = Cu, Fe) dielectric crystals was discovered.
The continuous phase transitions are observed in the crystals with incommensurate phase. They are transitions of parent-incommensurate phases (T_i); transitions between metastable states; incommensurate-commensurate phases (T_c). It was set that phase transition between parent and incommensurate phase is a continuous second-order phase transition with a critical index β = 0.5. The transition between metastable states is a continuous phase transition through the intermediate temperature region - incommensurate phase. The wave vector changes with the temperature here and wave vectorwe q* = q₁ - q₂ appears, where q₁, q₂ denote commensurate values of incommensurability wave vector of neighboring metastable states. It was shown that the phase transition between incommensurate and commensurate phases is a continuous phase transition.
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