Characteristic Features of the Modulated Phases in (MgGeF6\text{}_{6})·6H2\text{}_{2}O Crystals

The temperature dependences of the optical birefringence δ(Δ n$\text{}_{c}$) in (MgGeF$\text{}_{6}$)·6H$\text{}_{2}$O 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

Behaviour of the Optical Birefringence in Conditions of the "Viscous" Interaction

The temperature dependences of optical birefringence δ(Δn) for the principal cuts of [N(CH$\text{}_{3}$)$\text{}_{4}$]$\text{}_{2}$CuCl$\text{}_{4}$ 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

Manifestation of a Coexistence of the Long-Periodic Phase with the Incommensurate Phase

Electric and optical properties of the commensurate ferroelectric long-periodic phase in [N(CH$\text{}_{3}$)$\text{}_{4}$]$\text{}_{2}$ZnCl$\text{}_{4}$ and [N(CH$\text{}_{3}$)$\text{}_{4}$]$\text{}_{2}$CoCl$\text{}_{4}$ crystals were investigated to explain mechanism of the double hysteresis loops appearance

Influence of Mechanical Stress on the Birefringence Properties of (N(CH3\text{}_{3})4\text{}_{4})2\text{}_{2}ZnCl4\text{}_{4} Crystals

The influence of applied mechanical stress on the birefringence of (N(CH$\text{}_{3}$)$\text{}_{4}$)$\text{}_{2}$ZnCl$\text{}_{4}$ crystal in the temperature region of its incommensurate phase was investigated using Senarmonth's method (λ = 633 nm). The value of pinning force for the samples with different defectiveness was experimentally determined

Investigations of High Temperature Phase Transitions in (NH3\text{}_{3}c2\text{}_{2}H5\text{}_{5})2\text{}_{2}CuCl4\text{}_{4} Crystals

On the basis of dilatometric, optical, and dielectric investigations of (NH$\text{}_{3}$C$\text{}_{2}$H$\text{}_{5}$)$\text{}_{2}$CuCl$\text{}_{4}$ crystals the existence of phase transitions at T$\text{}_{1}$=364 and T$\text{}_{2}$=356 K was confirmed. Anomalous behaviour of the thermal expansion coefficient and optical birefringence around T$\text{}_{3}$=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

The Stochastic Mode of the Modulated Structure in [N(CH3\text{}_{3})4\text{}_{4}]2\text{}_{2}MeCl4\text{}_{4} Dielectric Crystals

Temperature behavior of the optical birefringence in the transitional regions of the incommensurate phase of [N(CH$\text{}_{3}$)]CuCl$\text{}_{4}$ 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$\text{}_{3}$)]MeCl$\text{}_{4}$ (Me = Cu, Fe) dielectric crystals was discovered

Dielectric and Birefringent Properties of [(CH3\text{}_{3})2\text{}_{2}NH2\text{}_{2}]5\text{}_{5}Cd2\text{}_{2}CuCl11\text{}_{11} Solid Solutions in the Region of Their Phase Transitions

On the basis of the optical and dielectric investigations of [(CH$\text{}_{3}$)$\text{}_{2}$NH$\text{}_{2}$]$\text{}_{5}$Cd$\text{}_{2}$CuCl$\text{}_{11}$ solid solutions the existence of phase transitions at T$\text{}_{1}$=175 K and T$\text{}_{2}$=117.5 K was confirmed. Both phase transitions were found to be shifted toward lower temperatures with respect to the corresponding transitions in the "host" [(CH$\text{}_{3}$)$\text{}_{2}$NH$\text{}_{2}$]$\text{}_{5}$Cd$\text{}_{3}$Cl$\text{}_{11}$ crystals. It was found that the proton conductivity in [(CH$\text{}_{3}$)$\text{}_{2}$NH$\text{}_{2}$ ]$\text{}_{5}$Cd$\text{}_{2}$CuCl$\text{}_{11}$ crystals is realized through the Grotthus mechanism. The investigations of the birefringent properties confirmed existence of the structural changes at T$\text{}_{0}$=320 K connected with the complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn-Teller distortion with temperature