Influence of Non‐Stoichiometric Defects on Optical Properties in LiNbO3

We present a band structure approach with a molecular dynamics cluster optimization which accounts for the various structural modifications related to the non‐stoichiometry of LiNbO₃ crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Particular role of the electron‐phonon contributions to the electrooptics coefficient is shown. Model calculations yield a large dependence of the electrooptis coefficient r₂₂ on the crystal composition, in agreement with the experimental data. The observed minimum of the r₂₂ coefficient versus the non‐stoichiometry is interpreted as originated from the non‐centrosymmetry in the electrostatic potential distribution around Nb‐O₆ clusters.     

Electric‐field‐induced internal deformation in piezoelectric BiB3O6 crystals

For the first time electric‐field‐induced atomic displacements (internal strains) in non‐ferroelectric polar BiB₃O₆ single crystal plates (point symmetry 2) were investigated using X‐ray diffraction technique. The intensity variations of selected Bragg reflections were collected for three different orientations of the applied external electric field vector with respect to the crystal lattice and used for calculating the microscopic structural response of BiB₃O₆. Due to the limited number of the reflections providing measurable changes in Bragg intensities we restricted ourselves in analyzing the shift of the B₃O₆ sublattice relative to the Bi one. In addition, we considered the deformation of the Bi‐O, B(1)‐O and B(2)‐O bond lengths and identified the [B(2)O₃] group as the most sensitive structural unit to an external electric perturbation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inhomogeneity of composition in near‐stoichiometric LiNbO3 single crystal grown from Li rich melt

A near‐stoichiometric LiNbO₃ single crystal has been grown by the Czochralski technique from a melt of 58.5 mol% Li₂O. Its composition homogeneity was assessed by measuring the UV absorption edge. It was found that the maximum composition difference is about 0.03 mol% in the radial direction and 0.05 mol% in the axial direction. Differential scanning calorimetry (DSC) analysis was performed on the powder from the synthesized raw material and the frozen melt after crystal growth. The analytical results indicate that, during crystal growth, the magnitude of lithium volatilization from the melt surface is more than the degree of segregation from the crystal. The volatilized lithium diffuses into the crystal to compensate for the lithium segregation in the LiNbO₃ crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some peculiarities of Ti in‐diffusion in lithium niobate

The process of one‐ and two‐dimensional Ti in‐diffusion in lithium niobate, LiNbO₃ (LN), single crystals of 〈x〉 and 〈z〉 orientations at 950‐1060°C in oxygen/water vapor medium had been studied. In the case of one‐dimensional diffusion, a flat diffusion front consisting of Ti solid solution in LN is formed. The process kinetics is described by Fick's equation for the case of a permanent source. In the case of two‐dimensional Ti diffusion, the diffusion zone contour acquires the shape close to elliptical. The diffusion rate in the tangential direction is about an order of magnitude higher than in the normal direction. A model qualitatively describing such nontrivial character of the diffusion process is suggested. The model is based on: a) incongruent lithium evaporation out of LN at high temperatures; b) low thermodynamic activity of Li in LiTiO₃ – product of Ti interaction with LN. These factors produce a significant nonlinearity of the process, since a strong chemical bond between Li and Ti in this phase leads to a mutual increase in their diffusive mobility. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Substituent and solvent effects on UV‐visible absorption spectra of liquid crystalline disubstituted biphenylcyclohexane derivatives – a computational approach

A computational approach has been carried out on liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of general formula R‐C₆H₁₀‐C₆H₄‐C₆H₄‐X with R: C₃H₇; X: H (BCH30) and R: C₅H₁₁; X: CN (BCH5CN) using the CNDO/S + CI and INDO/S + CI methods. These methods have been employed to calculate and analyze the spectral shifts, and absorbance measurements in UV‐visible range of the systems. The electronic transitions, absorption wavelength, HOMO (Highest Occupied Molecular Orbital), and LUMO (Lowest Unoccupied Molecular Orbital) energies have been calculated. Further, ultraviolet (UV) stability of the molecules has been discussed in the light of absorption wavelength and electronic transition oscillator strength (f). The effect of different solvent media and substituents on transition energies, oscillator strength, and other absorption parameters has also been reported. The present article provides valuable information regarding enhancing the UV stability of molecules by marinating their conductivity. Further, these absorption spectra will provide valuable data base for future computational studies, and the other experimental investigations. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radiation induced modification of impurity‐related point defects in crystalline quartz – a review

This article presents a short review of impurity‐related point defects in crystalline quartz and their radiation induced modifications.In particular, a discussion has been presented on some of the prominent aluminum related alkali and proton compensated centers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)