Stakhura V. The optoelectronic properties of uniaxially clamped incommensurate modulated Rb2ZnCl4 crystals

In the work the temperature, spectral and pressure dependences of birefringence ni of Rb2ZnCl4 crystals have been studied and the dependence of birefringence from uniaxial pressures has been found. The latter induce a new isotropic state in the crystal. Using the pressure dependences, the spectral and temperature dependences of the piezooptical coefficients are investigated. It has been found that they have small dispersion dependence and significant anomalies at phase transition temperature Тс = 192 К. It has been obtained that the uniaxial stresses shift the temperature of phase transitions on the temperature scale in different directions depending on the direction of compression. The temperature-pressure phase diagram of the crystal has been constructed. From which it has been found that incommensurate phase disappear and paraelectric- ferroelectric phase transition occurs at the pressure с  18,3 0,5 kbar and the temperature Т  28,3  0,1 К. The spectral, temperature and pressure dependences of the refractive indices ni() have been studied and it has been shown that the uniaxial compression does not change the properties of curves ni() and ni(Т), but change the quantities dni/d and dni/dТ. Pressure dependences of electron polarization і,, refraction R and parameters of ultraviolet oscillators (0і, В1і) have been calculated. It has been found that the uniaxial compression increases Rі and і. It was shown that increasing of ni because of the uniaxial pressure was mainly due to increasing of refractions (~65%) because of the change of band gap Eg and the long-wave shift of the ultraviolet absorption band maximum and the density of effective oscillators (~ 35%) of the Rb2ZnCl4 crystal. The band structure of Rb2ZnCl4 crystals has been calculated using two approximation of the exchange-correlation potential (generalized gradient approximation (GGA) and local density (LDA)) in the basis of plane waves. It has been found that energy levels of valence band have weak dispersion in the k-space, while the energy levels of conduction band have significant dispersion. The highest dispersion of energy states is localized near the center of the Brillouin zone, at Г point. The value of the smallest direct band gap (at Г point) has been determined. In the conduction band, 4 subbands has been found. The subbands are mainly formed by s-states of Rubidium and Zinc. These states are localized near the Z point of the Brillouin zone and hybridized with the p-states of Сhlorine. The atomic population of the components, the length and degree of population of the shortest atomic bonds in the Rb2ZnCl4 crystal have been calculated. It has been shown that the maximum population of Zn-Cl bond in ZnCl4 tetrahedron is 0.56, which confirms the high covalence of Zn-Cl bonds. On the basis of the dependences of the real and imaginary parts of the complex dielectric permittivity, using the Kramers-Kronig dispersion relations, the spectral dependences of the refractive indices nі(λ) and birefringence Δnі have been calculated and compared with experimental data. It has been established that the dispertion (∂n/∂λ < 0) and the character of anisotropy (na > nc > nb ) remain the same in both cases.