Dielectric Properties of Beta Barium Borate and Potassium Pentaborate Single Crystals

Single crystals of beta barium borate and potassium pentaborate (commonly known as β-BaB₂O₄ or BBO and KB₅O₈. 4 H₂O or KB5) were grown by high and low temperature solution growth methods, respectively. The dielectric constant and loss tangent were determined as a function of temperature. Frequency response of the dielectric constant and tan δ have been studied over a frequency range of 0.1 −100 kHz; it is found that both the dielectric constant and tan δ decrease with increase in frequency for both crystals. The dielectric constant and tan δ are found to increase with increase in temperature for both the crystals. Further, the dielectric constant and tan δ measured on different orientations of the grown crystals reveal the anisotropic nature of the crystals.     

Incorporation of alkali impurities into single crystals of barium metaborate β-BaB2O4

The impurities in single crystals of the low-temperature β modification of BaB₂O₄ grown from flux in the BaB₂O₄-Na₂O system have been studied. The β-BaB₂O₄ compound was examined by X-ray powder diffraction analysis. The appearance of scattering centers in the crystals correlates with the high content of sodium whose critical concentration is about 0.021 wt %. The effective distribution coefficient of sodium is 2.5–4.4 × 10^?3. The conditions for constitutional supercooling at the crystallization front are considered.     

Phase formation in the BaB2O4-BaF2 system

It is shown that the BaB₂O₄-BaF₂ system is quasi-binary with the following eutectics coordinates: 760°C, 59 mol % BaF₂, 41 mol % BaB₂O₄. Due to the intense pyrohydrolysis during the growth of ??-BaB₂O₄ crystals from the 55.6 mol % BaB₂O₄-44.4 mol % BaF₂ composition, the Ba₅B₄O₁₁ compound is formed in the system. This process leads to the cocrystallization of the ??-BaB₂O₄ and Ba₅B₄O₁₁ phases and impedes the formation of high-quality crystals.     

Morphology of β-BaB2O4 (BBO) Crystals in Relation to its Structure and Growth Conditions

β-BaB₂O₄ (BBO) crystals with well-defined morphology have been grown from Na₂O solutions using the top seeded solution growth (TSSG) method. The crystal morphology in relation to its structure and growth conditions has been studied in detail on the basis of crystallography and crystal chemistry. It is found that the morphological characteristics are related to the orientations of structural unit (B₃O₆)^3— anion rings in the crystal. On the other hand, the growth parameters may greatly affect the appearance of faces of the crystal, but the crystals still generally take trigonal in outline and have a diagnostic character of point group 3m. The observed morphology is in disagreement with BFDH and PBC analyses and is explained from the incorporation of the growth units on the faces and facets. Since the incorporation rates of the growth units are different on different faces, the boule habits with well-defined morphology are formed.     

Change of symmetry and rotation of thermal field as a new method of control of heat and mass transfer in crystal growth (by example of β-BaB<Subscript>2</Subscript>O<Subscript>4</Subscript>)

It is suggested to change the symmetry and rotation of thermal field as a method of contact-free control of the heat and mass transfer in crystal growth. By the example of growth of the low-temperature barium borate (β-BaB₂O₄) phase, a technically important crystal with nonlinear-optical properties, it is shown that the use of the suggested method allows one to grow larger crystals of a higher quality.     

Twinning Morphologies and Mechanisms of β-BaB2O4 (BBO) Crystal Grown by TSSG Method

The twinning morphology of β-BaB₂O₄ crystal grown by TSSG has been studied on the basis of crystallography. It was found that the faces R{101-2} and r{011-4} are often intergrown together by either r{011-4} intergrown in R{101-2} or in reverse, two individuals mostly take X(112-0) and occasionally take Y(011-0) as twin boundary; and that two individuals in the twin could take the same or opposite crystallographic polarity, this was identified by surface etched figures and the piezoelectric measurements. The formation of the twinned crystals are discussed from the crystallographic structural characteristics and the energetic considerations. The experimental factors such as crystal seed quality are suggested to reduce or eliminate the twin formation and to improve the crystal quality.     

Growth aspects of barium oxalate monohydrate single crystals in gel medium

Single crystals of barium oxalate monohydrate (BaC₂O₄.H₂O, BOM) were grown in pure form by controlled diffusion of Ba²⁺ using the gel technique at different temperatures. Starting from aqueous Ba²⁺ chloride (BaCl₂) and acetic acid (C₂H₂O₄) in gel, this method offers a low‐cost and an easiest alternative to other preparation methods for the production of barium oxalate bulky single crystals. The optimal conditions for the growth of BOM crystals in silica gel were found by investigating different growth parameters such as gel pH, gel aging and crystallization temperature. Irrespective of all such crystallization environments, growth rate of the crystals were initially less and then exhibited supersaturation effect leading to non‐linearity. Gel aging and temperature has profound effect on nucleation density that resulted less number of crystals of maximum size in the gel matrix. Perfect single crystals were grown on gels of higher pH. The macropore morphology and porosity was controlled by changing age of the gel. It has been found that temperature has a fabulous effect in controlling the nucleation density by altering the supersaturation conditions for the formation of critical nuclei. The entire growth kinetics informed that the grown crystals were derived by the one dimensional diffusion controlled process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature distribution in growing semi-transparent crystals (II). Comparison of theory with experiments

Calculated temperature difference between the liquid/solid interface and bulk of the melt was compared with the wide-angle scattering (celluae density) of Al₂O₃ and YAG crystals. Celluae free coloured crystals were grown using higher energy losses from the upper part of the crystal and the melt. Colourless crystals were successfully grown under opposite conditions. The results are in accordance with the calculated temperature differences which are proportional to the actual temperature gradient at the interface.     

Surface crystallization of β-BaB2O4 phase using a CO2 laser source

This letter describes decrystallization process of the β-BaB₂O₄ phase on the surface of 63BaO-26B₂O₃-11TiO₂ glassy sample when irradiated by a CO₂ laser beam for 30-40 s, using a laser power of ≈1.7 W. An analysis by optical microscopy of the irradiated region showed that the glass surface contained crystallites with the same morphology as those observed after traditional crystallization in an electric furnace. However, the crystal growth rate and the crystallite sizes appeared to be higher than those obtained by heating in an electric furnace. X-ray diffraction of the irradiated region showed a preferential orientation of the β-BaB₂O₄ crystallites on the glass surface.     

Single-crystal growth of the high-pressure phase B2O3 II

The technique and results of the hydrothermal single-crystal growth of the high-pressure phase B₂O₃ II are described. Transparent colorless crystals 450 × 450 × 150 μm in size have been grown under hydrothermal conditions (pressure 5 GPa, temperature range 1425–1025 K, and cooling rate ∼100 K/h).     

Crystallographic and dielectric properties of flux grown (B′B″: InNb, InTa, YbNb, YbTa and MgW) single crystals

Single crystals of PbIn_1/2Nb_1/2O₃ (PIN), PbIn_1/2Ta_1/2O₃ (PIT), PbYb_1/2Nb_1/2O₃ (PYN), PbYb_1/2Ta_1/2O₃ (PYT) and PbMg_1/2W_1/2O₃ (PMW) have been grown by the flux method. The PbO-based solvents were used. Transparent, light yellow and arrow like shaped PIN and PIT crystals of the perovskite structure were obtained. Small amounts of red and of octahedron habit PIN and PIT crystals of the pyrochlore type were simultaneously grown. In the case of PYN, PYT and PMW only the crystals of the perovskite structure have been grown. The transparent and brown PYN and PYT crystals of octahedron habit were obtained. The transparent, light yellow and of octahedron or truncated octahedron shape PMW crystals were grown. The crystals were characterised by X-ray and dielectric studies. They showed that as-grown PIN crystals are nearly disordered, exhibit the rhombohedral distortion of the pseudo-perovskite unit cell and reveal relaxor behaviour. The partially ordered PIT crystals show monoclinic distortion and undergo antiferroelectric–paraelectric phase transition. The PYN, PYT and PMW single crystals, characterised by chemical order in the B′/B″ ion sublattice, exhibit orthorhombic symmetry and undergo the first-order antiferroelectric–paraelectric phase transitions.     

Raman scattering study of crystalline and melting states of BaO·2B2O3

Melting and crystallization scenarios of barium tetraborate BaB₄O₇ (BaO·2B₂O₃) are studied in situ by Raman spectroscopy. It is shown that the scenario depends on the temperature–time history of melt. Crystallization conditions of the beta modification of barium tetraborate (β-BaB₄O₇) from a stoichiometric glass structure BaO·2B₂O₃ were investigated.     

Czochralski Growth of Potassium Lithium Niobate Single Crystals with Low Li Content and Their Characterization

Single crystalline and crack free potassium lithium niobate (KLN) single crystals with low Li content were grown by the Czochralski method. The crystal composition can be written as K_2.60Li_1.17Nb_5.44O₁₅ (=K_2.95Li_1.33Nb_6.17O₁₇) which contain relatively fewer Li ions than ferroelectric K₃Li₂Nb₅O₁₅ crystals. All experimental results show that the deficiency of the Li ions in the KLN crystals strongly influences their physical properties. Especially, the as‐grown crystals do not indicate any signature for a ferroelectric phase transition in contrast to the ferroelectric K₃Li₂Nb₅O₁₅ crystals. However, due to ionic conduction, the temperature dependence of the dielectric constant of such KLN‐2 crystals show a broad anomaly near 300°C. In addition, the existence of proton defects can be revealed by infrared absorption spectroscopy near 3500 cm^‐1 in as‐grown crystals.     

Crystal growth and characterization of tetragonal tungsten bronze FerroNiobates Ba2LnFeNb4O15

Tetragonal Tungsten Bronze Ferroelectrics (TTB) are currently revisited for the elaboration of new multifunctional materials. Recent studies on Ba₂LnFeNb₄O₁₅ ceramics (Ln=Nd, Sm and Eu) demonstrated that these materials display both ferroelectric and ferromagnetic behaviors at room temperature due to the presence of barium hexaferrite as a secondary phase. In this paper, we report for the first time the growth of Ba₂LnFeNb₄O₁₅ single crystals (Ln=La, Pr, Nd, Sm and Eu) from high temperature solution using LiBO₂ flux. Inclusion free millimeter-sized single crystals were successfully grown. Structural and dielectric characterizations were performed and chemical analysis confirmed that the TTB matrix is reluctant to accommodate small rare earth as it was observed for ceramics. A paramagnetic behavior was observed for all compositions, which corroborates the composite nature of the TTB ceramic multiferroics at room temperature, while dielectric measurements on Ln=La and Pr crystals confirmed their relaxor behavior.     

Growth of K(DxH1 − x )2PO4 single crystals from solutions in the K2O-P2O5-(D, H)2O system

Based on the analysis of the K₂O-P₂O₅-D₂O solubility phase diagram, the optimum conditions of KD₂PO₄ crystallization—the compositions of mother solutions and the temperature range of crystallization—in the KH₂PO₄-D₂O system have been determined. The technique of K(D_xH_{1 ? x} )₂PO₄ growth is developed. The DKDP single crystals with deuterium concentration up to 88 wt % are grown on DKDP seeds from KH₂PO₄ solutions in D₂O by the method of temperature decrease.     

Crystal structure,dielectric properties of (K0.5Na0.5)NbO3 single crystal grown by flux method using B2O3 flux

Lead free piezoelectric single crystals of sodium potassium niobate (K_0.5Na_0.5)NbO₃ (KNN) were grown by high‐temperature solution method using two different fluxes; one with a mixture of NaF and KF and other with addition of B₂O₃ along with the mixture. In the present study, the growth of KNN crystals without B₂O₃ flux and the same with B₂O₃ flux were compared. It was found that additions of small amounts of B₂O₃ lowered the melting temperature of the solid solution and enabled better dielectric properties. Phase analysis showed that all samples were crystallized in pure orthorhombic perovskite phase. AFM morphological studies showed that the addition of B₂O₃ flux increased the roughness of the grown crystal. Further, addition of B₂O₃ flux slightly decreased the orthorhombic to tetragonal phase transition temperature T_(O—T) and the Curie temperature T_C. The ferroelectric behaviour of KNN single crystal has been investigated at room temperature. The crystal grown using B₂O₃ flux exhibited a remanent polarization (P_r) ∼ 32 μC/cm² and coercive field (Ec) of ∼11.8 kV/cm whereas the crystal grown without the use of B₂O₃ flux had a remanent polarization (P_r) ∼ 36 μC/cm² and coercive field (Ec) of ∼14.6 kV/cm.     

Optical properties of the germanate melilites Sr2MgGe2O7, Sr2ZnGe2O7 and Ba2ZnGe2O7

Refractive indices and their dispersion in the wavelength range from 365 nm to 2325 nm and transmission ranges of the tetragonal melilite‐type germanates Sr₂MgGe₂O₇, Sr₂ZnGe₂O₇ and Ba₂ZnGe₂O₇ were determined. The uniaxial positive crystals Sr₂MgGe₂O₇ and Ba₂ZnGe₂O₇ both offer the possibility for phase matched second harmonic generation, a detailed analysis of phase matching conditions is given. The refractive indices of Sr₂ZnGe₂O₇ show an isoindex (isotropic) point at 467 nm. The investigation was performed on Czochralski grown large single crystals. The crystal structure of all three germanates were determined by means of X ‐ray diffraction. The results corroborate unmodulated melilite‐type structures at room temperature. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of single crystals of NaH2PO4 (NaDP), NaH2PO4 · H2O (NaDP · H2O), and NaH2PO4 · 2H2O (NaDP · 2H2O) sodium dihydrogenphosphate based on the analysis of the Na2O-P2O5-H2O phase diagram

The crystallization conditions for the NaH₂PO₄, NaH₂PO₄ · H₂O, and NaH₂PO₄ · 2H₂O solid phases have been established from the analysis of the phase diagram of solubility of the ternary Na₂O-P₂O₅-H₂O system in the temperature range from 0 to 100°C. Based on these data, the methods for growing sodium dihydrogenphosphate single crystals of the above compositions are developed. The initial components for preparing mother solutions were H₃PO₄ and NaOH solutions taken in certain weight ratios. For the first time, NaDP, NaDP · H₂O, and NaDP · 2H₂O single crystals were grown on a seed by the method of temperature decrease. The habits of the NaDP and NaDP · H₂O single crystals are determined. __________ Translated from Kristallografiya, Vol. 47, No. 5, 2002, pp. 937–944. Original Russian Text Copyright ? 2002 by Soboleva, Voloshin.     

Growth of ytterbium tartrate trihydrate crystals in silica and agar‐agar gels and their characterization

Single crystals of ytterbium tartrate trihydrate have been grown by gel method using silica and agar‐agar gels as media of growth. The medium of growth influences the morphology of grown crystals, silica gel yielding single and polycrystalline in the form of spherulites whereas agar‐agar gel leading to growth of single and twinned crystals. Materials grown as single crystals have been characterized by using optical and scanning electron microscopy (SEM), EDAX, XRD, FT‐IR, CHN and thermogravimetric techniques. The stoichiometry of the grown single crystals is suggested to be Yb(C₄H₄O₆) (C₄H₅O₆).3H₂O. The FT‐IR spectrum shows the presence of singly as well as doubly ionized tartrate ligands. Results of thermal analysis indicate that the material is thermally stable up to a temperature of 200 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature distribution in growing semi-transparent crystals (III). Conical liquid/solid interface

Temperature distribution in Al₂O₃ and YAG crystals grown by Czochralski method and in the melt layer adjacent to the liquid/solid interface of the conical shape was computed. The results were compared with the quality of the crystals. Using a highly absorbing material, temperature difference near the deeply submerged sharp interface decreases from the edge to the centre of a crystal and it is relatively great and unalterable in the case of a shallowly submerged blunt interface. Sharp interface show the crystals grown from the melt of the same composition (Al₂O₃ or accurately “stoichiometric” YAG), whereas the blunt one is typical of YAG grown from the melt of the “non-stoichiometric” composition, because its solidifying point is below the melting point of YAG phase.