Phase states in rotationally distorted perovskites

The thermodynamic and crystallochemical conditions determining the types of the rotational phase transitions and their sequence in cubic perovskites are established. It is shown that a decrease in the tolerance factor and a change of the A-cation and its charge stimulates the softening of the rotational-vibrational mode M ₃. As an example, the compounds A _x Na_{1 ? x} NbO₃(A = K, Li) are considered.     

Crystal growth and structure of pure and rare-earth doped barium sodium niobate (BNN)

Through the exploitation of the corresponding ternary phase diagram, the barium sodium niobate (BNN) crystal growth by several techniques: Czochralski, Laser Heated Pedestal Growth and Micro-Pulling Down is surveyed. It is pointed out that the fiber crystal growth contributed to obtain important advances on the crystal structure, doping by rare-earth ions and growing behavior leading to what can be considered as an attractive new material. The further growth of high optical quality bulk crystals could now lead to the commercial development of this material with outstanding non-linear optical properties.     

Phase evolution on heat treatment of sodium silicate water glass

Heat treatment of sodium silicate water glass of the nominal composition Na₂O/SiO₂ = 1:3 was carried out from 100 °C up to 800 °C and the advancement of the resulting phases was followed up by powder X-ray diffraction, scanning electron microscopy and thermogravimetry along with differential thermal analysis. The water glass, initially being an amorphous solid, starts to form crystals of β-Na₂Si₂O₅ at about 400 °C and crystallizes the SiO₂ modification cristobalite at about 600 °C that coexists along with β-Na₂Si₂O₅ up to 700 °C. At 750 °C Na₆Si₈O₁₉ appears as a separate phase and beyond 800 °C, the system turns into a liquid.     

Piezo-optic surfaces of lithium niobate crystals

A method of construction of the spatial distribution of the piezo-optic effect in crystals is proposed. A particular case of this method is the known technique of construction of indicator surfaces of the piezo-optic effect. The essence of the proposed method consists in determining the difference in the radius vectors of the optical indicatrix perturbed by stress and the free optical indicatrix. It is shown that this difference in the radius vectors is mathematically identical to the law of transformation of the piezo-optic tensor during the rotation of the coordinate system.     

Phase states of the gypsum thermal-annealing derivatives according to electron spin resonance spectra

The electron spin resonance (ESR) spectra of SO ₃ ^? and SO ₂ ^? radical ions with a linewidth ΔH ≈ 2.7 G and SO ₃ ^? (A ₁) and SO ₃ ^? (A ₂) centers with superhyperfine splitting due to the interaction with protons in platelike gypsum single crystals under X-ray irradiation have been analyzed at 25°C. Dehydrated regions with a radius >4 Å are revealed in gypsum. The ESR spectra of SO ₃ ^? radical ions and atomic hydrogen with ΔH ≈ 0.3 G are found in the products of isothermal annealing of gypsum kept for 30 min after X-ray irradiation at 25°C. The dependences of the intensities of these spectra on the annealing temperature are studied in the range of 100–450°C. The temperature range of formation of α- and β-phase states of bassanite and γ-anhydrite are determined. The process of residual water redistribution between the channel systems of the α- and β-phase types of γ-CaSO₄ in gypsum thermal derivatives is established.     

Growth and analysis of barium-sodium niobate crystals

The conditions of growth, monodomenization, and detwinning are considered in order to obtain optically perfect barium–sodium niobate crystals (BSN). Optical homogeneity and that of chemical composition are investigated. It is shown that possible variations of composition in a crystal's volume are below 1%. Thermal E and optical E_g° widths of the forbidden zone are determined in the absorption spectra.     

Linear defects in single-crystals of lithium niobate

The method of studying the dimensional distribution of the density of dislocations revealed the effect of different stages of process of growth upon the structural perfection of crystals of LiNbO₃, grown by the Czochralski's method. Probable reasons for development of dislocations are discussed.     

Phase transitions and macroscopic properties of NaNO3 embedded into porous glasses

The crystal structure and dielectric response of nanocomposite materials on base of porous glasses with average pore diameters of 320, 46 and 7 nm with embedding sodium nitrate have been studied by neutron diffraction and dielectric spectroscopy in low and high temperature phases up to melting. In porous glasses with 46 and 7 nm pores NaNO₃ forms dendrite nanoclusters with "diffraction" sizes of 50(2.5) and 20(2) nm. Decreasing of particle sizes results in decreasing of T_c (temperature of order-disorder orientational transition) and T_melt and in smearing of structure phase transition. The values of critical exponent β for orientational transition are estimated from temperature dependences of intensities of superstructure elastic peaks for these three types of nanocomposite materials.     

Investigation on photorefractive properties of doped lithium niobate

Cu:LiNbO₃ crystal and Fe:Cu:LiNbO₃ crystals were grown by the Czochralski method from congruent melt. The OH^‐ absorption spectrum of doped lithium niobate crystals was measured. The photorefractive properties of doped crystals were studied by the two‐wave coupling method. The results of the two‐wave coupling experiments showed that as the concentration of doping ions increased, the diffraction efficiency and the dynamic range enhanced, the holographic response time shortened. The recording time of Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal is only a tenth of that of Cu(0.05wt%): LiNbO₃ crystal. Among all samples, the dynamic range of the Fe(0.10wt%): Cu(0.10wt%): LiNbO₃ crystal was the most largest (up to 40.78). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of nanocrystalline lithium niobate powders at low temperature

A facile route to prepare lithium niobate (LiNbO₃) powders was proposed by an alternative solid‐state method. Stoichiometric Li₂C₂O₄ and ammonium niobium oxalate were mixed with small amounts of water and then dried at room temperature. It was demonstrated that Li[NbO(C₂O₄)₂]·n H₂O intermediate was produced by an ion‐exchange reaction. Pure LiNbO₃ powders were successfully synthesized by heating the intermediate at 500, 600 and 700 °C for 3 h. X‐ray diffraction (XRD), scanning electron microscopy (SEM), Fourier‐transform infrared (FTIR) spectroscopy, UV‐Vis diffuse reflectance (UV‐Vis) spectroscopy and thermogravimetric (TG) analysis were used to characterize the precursor compound and as‐prepared samples. XRD results reveal that all the products are identified as hexagonal structure with high relative crystallinity (>87%). The particle size is found to be about 40 nm for the mixture calcined at 500 °C according to XRD data, which is in good agreement with SEM data. The as‐prepared LiNbO₃ powders by this method are high quality according to FTIR spectra. (Li_0.996Nb_0.005)Nb_0.999O₃ phase was formed when the calcination temperature was raised to 800 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of textured porous Al2O3 ceramics by slip casting in a strong magnetic field and its mechanical properties

The textured porous Al₂O₃ ceramics were prepared by slip casting in a strong magnetic field of 6 T and subsequently sintering. The c axis of Al₂O₃ grain was oriented parallel to the direction of the magnetic field and the textured porous microstructure with plate‐shape grains was formed. The porosity of textured porous Al₂O₃ ceramic was 30.37% and the relative density reached 66.29% when the sintering temperature is 1600°C. The textured porous Al₂O₃ green body showed the linear shrinkage anisotropy. The bending strength of the textured porous Al₂O₃ ceramics depended on the alignment direction of plate‐shape grains.     

Bridgman growth and properties of potassium lithium niobate single crystals

Transparent KLN crystals 10mm in diameter and 25 to 45mm in length have been grown by the modified vertical Bridgman technique from different melts in the range of 3035mol% K₂O, 1723mol% Li₂O and 4350mol% Nb₂O₅. The growth conditions are a growth rate of less than 0.25 mm/hr, temperature gradient in solid-liquid interface of 23 °C/mm and growth direction of <110>. As-grown KLN crystals have tetragonal tungsten bronze structure. Most of the as-grown crystals do not crack when cooling through the paraelectric/ferroelectric phase transition. 180° domain structures are observed after the KLN crystal was etched in boiling 2HNO₃:Hf. Dielectric properties and transmission spectrum of the as-grown KLN crystals are measured.     

Modeling of cluster formation in nonlinear optical lithium niobate crystal

The processes occurring during the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of lithium niobate (LiNbO₃) crystal, have been qualitatively modeled in dependence of the phase composition. The modeling results are compared with the data obtained within vacancy models. It is shown that the cluster structure constructed along the crystallographic Y axis is most ordered, while that constructed along the polar Z axis is least ordered. The largest spread in the ratio R = Li/Nb is observed in the direction of the Z axis.     

GeO2/SiO2-glasses from gels to increase the oxidation resistance of porous silicon containing ceramics

GeO₂/SiO₂-glasses with relatively high expansion coefficients (between 5 and 7.3 × 10^?6 °C^?1) were prepared by hydrolysis of metal alkoxides (tetramethoxy silane+tetraethoxy germane as well as mixtures of the silane with GeO₂-powder) with subsequent heat treatment up to 1000°C. It is shown that the preparation of thin films on glasses and ceramics is possible but also that this technique can be used for liquid infiltration of porous SiC- and Si₃N₄-materials to increase the oxidation behaviour in the temperature range 1000 to 1400°C. In the case of molybdenumdisilicide as a ceramic material it is shown that the catastrophic inner oxidation in the low temperature range (600 to 800°C) of porous specimens (tested with plasma-sprayed MoSi₂-layers on refractory metals) can be hindered if these materials are infiltrated and heat treated.     

Relationship between the optical inhomogeneity and microdomain structure of lithium niobate

A model for the occurrence of optical inhomogeneity in lithium niobate single crystals in the regions of localization of microdomains with charged walls is considered.     

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)     

Laser heated pedestal growth of potassium lithium niobate for UV generation

Potassium lithium niobate (KLN) is a nonlinear optical material with a high nonlinearity. It has the potential to improve the performance and reduce the cost of blue and UV lasers. KLN crystals are not commercially viable because growth by traditional techniques is not possible. In an effort to develop commercially viable KLN, single crystals of the material were grown by the laser heated pedestal growth method (LHPG) with compositions of x=0.02, 0.06 and 0.2 following K₃Li_2?xNb_5+xO_15+2x. Noncritical phase matching at 20 °C for previously unreported compositions of x=0.02 and 0.06 was measured at 795 nm and 805 nm, respectively. Overall, the results suggest that single crystal KLN can be used for SHG into the UV region of the spectrum and can be developed into a commercially viable nonlinear optical material.     

Octagonal crystals of strontium barium niobate (SBN: 61)

Strontium barium niobate (SBN: 61) crystals, as grown by the Czochralski technique, are normally round in cross section and the circumference is ringed by 24 facets. We have grown SBN: 61 crystals with low axial temperature gradient conditions and have obtained crystals which are octagonal in cross section.     

Complex study of the structural and optical homogeneity of lithium niobate crystals

Methods of Raman spectroscopy, laser conoscopy, optical microscopy, and electron spin resonance have been used to study the photorefractive properties and structural and optical homogeneity of the following lithium niobate (LiNbO₃) crystals: nominally pure crystals of congruent composition (LiNbO_3con); LiNbO₃:Cu[0.015 wt %] crystals grown from a melt of congruent composition and nominally pure crystals of stoichiometric composition grown from a melt with 58.6 mol % Li₂O (LiNbO_3st). A small deformation of optical indicatrix and regular microdomain structures of fractal type are revealed for the LiNbO₃:Cu[0.015 wt %]; the microdomain structures may be due to the nonuniform impurity incorporation into the structure. It is shown that oxygen octahedra in the LiNbO₃:Cu[0.015 wt %] crystal are deformed in comparison with the octahedra in LiNbO_3st and LiNbO_3con crystals and that the main and impurity cations are clusterized along the polar axis. It is established that the LiNbO₃:Cu[0.015 wt %] crystal exhibits photorefractive properties not only due to the presence of intrinsic defects with localized electrons, as in the case of LiNbO_3st, but also due to the charge exchange in copper cations (Cu²⁺ → Cu⁺) under illumination.     

Crystallization kinetics of lithium niobate films from limited volume of melt solution

The growth kinetics of LiNbO₃ films from a limited volume of melt solution is observed. We developed crystallization models describing the character of mass transfer in the liquid phase for isothermal and non-isothermal epitaxy. Analytical expressions have been derived connecting the film thickness with the system growth parameters. The experimental values being in good agreement with the calculated ones.