Cover Picture: Cryst. Res. Technol. 1/2010

The cover picture shows a photograph of NaBi(WO₄)₂ crystals grown from a sintered charge in resistive furnace (left), a sintered charge in inductive furnace (middle) and a re‐crystallized charge in inductive furnace (right). (see pages 18–24). (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and Properties of Potassium Holmium Double Tungstate KHo(WO4)2

Potassium holmium tungstate KHo(WO₄)₂ crystal, as other tungstates, shows the high temperature irreversible structural phase transition. Owing to this, KHo(WO₄)₂ single crystals were grown due to spontaneous crystallisation with use of the High Temperature Solution Growth technique, which allows to lower the temperature of crystallisation below the temperature of the phase transition. K₂W₂O₇ was used as a solvent. It provides a very wide temperature range of crystallisation and does not introduce additional impurities into the melt. The starting flux contained 20 mol% of KHo(WO₄)₂ dissolved in K₂W₂O₇. It was found that potassium holmium double tungstate is pleochroic. The two different optical spectra: one spectrum for electrical vector of linearly polarised light parallel to optical Y axis (main spectrum) and second one for electrical vector perpendicular to Y axis were measured.     

Crystal growth and structure of KBi(WO4)2 single crystals

Potassium bismuth tungstate [KBi(WO₄)₂] single crystals have been grown by the top‐seeded solution growth technique. Bulk crystal with dimensions up to several centimeters is obtained for the first time. Several self‐flux systems have been used for the growth from the solution and the experiments using K₂W₂O₇ as a solvent are detailed. Powder and single crystal X‐ray diffraction of this crystal are reported. The structure refinement shows that KBi(WO₄)₂ crystallizes in the monoclinic space group C2/c, with a=10.837(3), b=10.586(3), c=7.622(2)Å, β=130.860(3)°, V=661.4(3)ų, and Z=4. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase formation in molten system (Na/K)2O‐TiO2‐P2O5. Crystal structures of NASICON and langbeinite‐related phosphates (K/Na)1+xTi2(PO4)3 (x = 0 and 0.357)

Crystallization of high temperature self‐flux of system Na₂O‐K₂O‐TiO₂‐P₂O₅ was investigated at different molar ratios (Na+K)/P = 0.9; 1.0 or 1.2 and Na/K = 1.0 or 2.0 over the temperature range 1000–650°C. The conditions of formation of complex phosphates K_0.10Na_0.90Ti₂(PO₄)₃ (NASICON‐related) and K_0.877Na_0.48Ti^ІІІ_0.357Ti^ІV_1.643(PO₄)₃ (langbeinite‐related) have been established. The new obtained compounds were investigated using FTIR‐spectroscopy, powder and single crystal X‐ray diffraction and optical microscopy methods. The influence of alkaline metal nature on the structure formation of complex phosphates in the high temperature self‐fluxes is discussed.     

Growth of single-crystalline sodium titanate and sodium tungstate one-dimensional nanostructures: Bio-inspired approach using oyster shell

Using oyster shell, single-crystalline Na₂Ti₆O₁₃ and Na₂W₄O₁₃ one-dimensional nanostructures (ODNS) were simply synthesized from granular TiO₂ and WO₃ particles by thermal heating at 700 °C. The oyster shell was used as a source of both sodium and carbon dioxide, which have an effect on unidirectional growth of single-crystalline Na₂Ti₆O₁₃ and Na₂W₄O₁₃ from TiO₂ and WO₃. The growth of sodium metal oxide ODNS from metal oxide particles was reproduced by replacing oyster shell with chemical reagents (NaCl, CaCO₃) or gas (CO₂), allowing identification of the main factors for the growth of ODNS. The characteristics of biological material (i.e., oyster shell) lend certain advantages in the fabrication of sodium metal oxide ODNS: (a) low processing temperature with high yield; (b) an eco-friendly method; and (c) no requirement of additional processes such as separation or washing. This work suggests a new approach for using biological material for the crystal growth of nanostructured materials.     

Crystallization of monoclinic WO3 in tungstate fluorophosphate glasses

Tungstate fluorophosphate glass compositions with high WO₃ concentration were prepared in the ternary system (80−0.8x)NaPO₃-(20−0.2x)BaF₂-xWO₃ with x = 40,50 and 60 mol%. Transparency decreases as WO₃ concentration increases but can be improved by addition of oxidizing systems such as CeO₂ or Sb₂O₃/NaNO₃. Characterizations by thermal analysis (DSC) point out that an increase in the amount of WO₃ results in a higher glass transition temperature. In addition, such compositions are very stable against devitrification since samples containing 40% and 50% of WO₃ donot even exhibit the expected crystallization event. In these samples, the stability against crystallization decreases with the WO₃ content and vitreous sample containing 60% of WO₃ exhibits an endothermic event around 620 °C due to crystallization of monoclinic WO₃ phase. In these glasses, it was shown that the nucleation stage can be induced by thermal-treatment when external nucleating agents such as Ti or Sb are used. Finally, gold-doped samples exhibit a higher crystallization tendency and monoclinic WO₃ phase can be grown in such glasses.     

Microwave hydrothermal synthesis and photocatalytic properties of ZnWO4 nanorods

Cd²⁺‐doped ZnWO₄ nanorods have been synthesized at 200 °C with microwave hydrothermal method, using Zn(NO₃)₂·6H₂O, Na₂WO₄·2H₂O and CdCl₂ as raw materials. The effects of Cd²⁺ doping contents on the structure and morphology of the product were studied. The results show that Cd²⁺ doping into the crystal lattice of ZnWO₄ nanopowder makes the powder orientationally grow along (010), (110) and (200) crystal planes to form the nanorods. The bigger Cd²⁺ doping contents are, the more obviously ZnWO₄ nanorods grow. Meanwhile, the nanopowder is gradually transformed from monoclinic phase into the orthogonal phase. As the charge transference medium between the interfaces, Cd²⁺ restrains the combination of holes and electrons. After doped, the photocatalytic properties of ZnWO₄ nanorods are increased. When Cd²⁺ doping content is 20%, the Cd²⁺‐doped ZnWO₄ nanorods showed the highest degradation rate up to 98% in 2 h.     

Thermal “order‐disorder” behaviour in (Na1‐xKx)4B8O14 solid solutions investigated by X‐ray powder diffraction

In the potassium‐rich part of the binary system Na₄B₈O₁₄‐K₄B₈O₁₄ solid solutions have been found that can be described with the formula (Na_1‐xK_x)₄B₈O₁₄ with 0.45 ≤ x < 1.0. The crystal structures of (Na_0.25K_0.75)₄B₈O₁₄ and (Na_0.45K_0.55)₄B₈O₁₄ were refined at room temperature by the Rietveld method. The solid solutions crystallize like K₄B₈O₁₄ in the triclinic crystal system, space group P ‐1, with K partially substituted for Na. An ordered distribution of the alkali atoms over the four cation sites at room temperature has been discovered. The structure of (Na_0.25K_0.75)₄B₈O₁₄ was also refined for data collected at 300 and 500 °C. The refinements show that sodium and potassium atoms are less ordered at higher temperatures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical properties of KGd(WO4)2 single crystals studied by Brillouin spectroscopy

The results of a study of KGd(WO₄)₂ single crystals by Brillouin scattering method are presented. The Brillouin spectra for the acoustic phonons propagating in the [100], [010], [001], [110] and [101] directions, taken in 90° and 180° scattering geometry at room temperature have been recorded. Using the Brillouin spectroscopy the refractive indices for some directions of KGd(WO₄)₂ crystals were determined. The refractive indices were also calculated for the same directions by the rotation transformation of the principal optical axes of the optical indicatrix to the crystallographic directions of KGd(WO₄)₂ crystals. Moreover, some anomalies of the acoustic phonons propagating connected with birefringence of KGd(WO₄)₂ crystals were discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature gradient controlled growth and optical properties of Er:BaY2F8 crystals

Single crystals of Erbium (Er) doped BaY₂F₈ have been obtained by the temperature gradient technique (TGT). No‐seed‐grown crystal of Er:BaY₂F₈, with the dimensions up to several centimeters, was obtained by self‐crystallization. The optimizations of various growth parameters were systemically investigated. The results indicated that the temperature gradient of 6‐7 K/mm and the cooling velocity less than 6 K/h were suitable for the crystal growth. The XRD data and the investigations on the growth striations by a stereo polarization microscope displayed that the [001] direction is the dominating direction for the crystal growth. The crystal grown by TGT often cracks along with the (100) plane, which is caused by the excessive decrease of the temperature during the crystal growth, for there is a rapid change in the thermal expansion curve of the BaY₂F₈ crystal in the temperature range from 800 °C to 900 °C. The spectral properties of Er:BaY₂F₈ single crystals have been studied and the effects of frequency up‐conversion of the crystals are reported. Spectral data suggest that the quality of Er:BaY₂F₈ crystal obtained by TGT method is good and the crystal has the potential application in laser devices. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exposure of high index facets in KDP crystals

A series of KDP crystals doped with WO₄^2‐ additive was grown by using rapid growth method. WO₄^2‐ additive inhibited the growth of (100) faces and made (100) faces tapering. With high additive concentration, two adjacent (100) faces twisted towards their intersecting crystal edge and (100) faces became twisting. Moreover, high index facets of (512) were exposed when the additive concentration was 500 ppm. The element analysis suggested that WO₄^2‐ was prone to being incorporated with (100) face. The growth kinetics of (100) face testified that WO₄^2‐ played an inhibited role. The AFM investigation on (100) face indicated WO₄^2‐ could impede step bunching. High concentration of WO₄^2‐ could kink the terrace of the macro‐steps and create another series of macro‐steps. The motion direction of the new creative steps was vertical to that of the original steps. When the motions of the vertical steps were both inhibited by WO₄^2‐, the steps would become tapering and (100) face would twist, which ultimately led to the exposure of (512) facets. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Proximity between reacting partners for diffusion-controlled growth of barium tungstate crystals from sodium tungstate melts

The separation distance (d₁₂) between a diffusing particle and its host necessary for a successful diffusion was estimated for diffusion-controlled crystal growth of BaWO₄ from Na₂WO₄ melts. Such distances slightly increased with increased cooling rates (R_T) and crystallization temperatures (T₀). The energy (E), enthalpy (ΔH_a), entropy (ΔS_a) and free-energy (ΔG_a) of activation, and the probability factor (P) were also estimated. These parameters did not change with the changes in R_T and T₀.     

Physical properties of self-flux and WO3-containing solutions useful for growing type III KGd(PO3)4 single crystals

Density, surface tension and dynamic viscosity of self-flux and WO₃-containing solutions useful for growing type III KGd(PO₃)₄ single crystals have been measured at temperature near the saturation temperature. The thermal behavior of these physical properties has also been studied. Solutions containing WO₃ show a higher density than self-flux solutions and the density decreases linearly when the temperature increases, in the two cases. Near the saturation temperature, self-flux solutions present a surface tension slightly lower than that of WO₃-containing solutions. The dynamic viscosity of WO₃-containing solutions is slightly lower than that of self-flux solutions when this property was measured at the same temperature. We observed that, in WO₃-containing solutions, the saturation temperature is lower than in self-flux solutions. Thus, at the growth temperatures, the two solutions present dynamic viscosities only slightly different, so we expect that the introduction of WO₃ up to 10 mol% in the growth solution does not represent any important improvement in its hydrodynamics and this small change does not compensate for the possibility of introduction of tungsten impurities in the crystal structure affecting the physical properties expected for these crystals. Taking into account the values measured for these physical properties, we choose the initial conditions for growing type III KGd(PO₃)₄ single crystals from self-flux solutions.     

Single crystals growth and absorption spectra of Cr-doped Al2−xInx(WO4)3 solid solutions

The growth conditions of pure and Cr³⁺-doped Al_2−xIn_x(WO₄)₃ single crystals, using top-seeded solution growth (TSSG) technique, have been studied. A series of experiments have been performed at different In concentrations, x=0.0, 0.3, 0.6 and 1.0, as well as at different concentrations of Cr³⁺ (0.0, 0.1, 0.2, 0.5 and 1.0) in at% with respect to the initial total concentration of Al and In in the starting solutions. The basic parameters of the crystal growth are varied over a wide range: seed orientation, speed of rotation, axial and radial temperature differences in the solution and the solution cooling rate. The investigated relations between the basic defects in the crystals and these parameters result in determination of the optimal conditions for growth of defect-free crystals. Distribution coefficients of Al, In and Cr have been determined, so the growth of crystals with given compositions is possible. Values of Dq/B (crystal field strength) for the various crystal compositions are calculated from the optical absorption spectra. The calculated values show that the discussed solid solutions have weak crystal field and are suitable for media with broadband emission spectra.     

Investigation of the Thermal Conductivity of Tungstate Crystals

Thermal conductivities of MWO₄ (M = Ca, Cd, or Ba), NaGd(WO₄)₂:1 at % Er, NaGd(WO₄)2:2 at % Yb, and NaLa_0.5Gd_0.5(WO₄)₂:2 at % Nd single crystals have been experimentally investigated in the temperature range of 50–300 K.     

Coarse crystal layer growth and liquid entrapment study with gradient freeze technology

The coarse crystal layer growth and liquid entrapment processes were investigated with gradient freeze technology in this paper. The research system was hemihydrate phosphoric acid (H₃PO₄·0.5H₂O) crystal‐phosphoric acid aqueous solution. The distribution coefficients of ions (Ca²⁺, Fe³⁺ and Na⁺) in this system were measured. The effect of supercooling degree gradient on layer growth and the effect layer growth rate on ions redistribution were studied. The result indicated that the layer growth rate increased with supercooling degree gradient as an exponential curve. The distribution coefficient tended to increase as an approximate ‘S’ curve when coarse crystal layer growth rate increased. The ‘three region’ theory was applied to explain this phenomenon. Each ion's diffusion parameter was obtained, which contributed to explain the separation differences between different ions. The work in this paper also indicated that layer crystallization was an effective separation technology for electronic grade phosphoric acid preparation.     

Contribution to the study of elastic properties of KGd(WO4)2:Er3+ single crystals by Brillouin spectroscopy

In this paper we present the study of the acoustic phonons propagating in Er³⁺‐doped KGd(WO₄)₂ single crystals by Brillouin spectroscopy. For the investigated crystals the velocities of the longitudinal and transverse acoustic phonons [100], [010], [001], [101] and [110] have been determined. Moreover, the values of the elastic constants: C₂₂, C₄₄ and C₆₆ of Er³⁺‐doped KGd(WO₄)₂ single crystals have been estimated. It was revealed that the presence of the Er³⁺‐ions in KGd(WO₄)₂ crystals, for the used doping concentration 1 at% does not influence their elastic properties. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Elastic properties of KGd(WO4)2:Ho3+ single crystals studied by Brillouin spectroscopy

The analysis some of the acoustic phonons propagating in pure and Ho³⁺‐doped KGd(WO₄)₂ single crystals in the GHz frequency range by Brillouin scattering method has been presented. For investigated crystals the velocities of the longitudinal and transverse acoustic phonons [100], [010], [001], [101], [101] and [110] have been determined. Moreover, the value of elastic constants C₂₂, C₄₄ and C₆₆ of pure and Ho³⁺‐doped KGd(WO₄)₂ single crystals have been estimated. It was revealed that the presence of the Ho³⁺‐ions in KGd(WO₄)₂ crystals, for the used doping concentration 1 at% and 8 at%, does not influence their elastic properties. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization Kinetics of Silicalite-1: Use of Tetrapropylammonium Bromide and Chloride as Templates

Silicalite-1, the aluminum-free end number of the ZSM-5 zeolites, was synthesized from a batch composition of 3.25 Na₂O · 40.0 SiO₂ · 552 H₂O · 2.00 TPA by using tetrapropylammonium bromide (TPA-Br) or chloride (TPA-Cl) as templates in the temperature range of 100 to 175 °C in a batch system. Conversion of silica in the starting batch composition into silicalite-1 in the product was followed quantitatively. The activation energies of nucleation and crystallization were determined as 37.2 and 66.5 kJ/mol, respectively. The use of TPA-Cl as the template instead of TPA-Br results in longer induction period for crystallization to start and a larger crystal size in product.     

Growth and characterization of ferroelectric‐ferroelastic Tb2(MoO4)3 crystals

Transparent and nearly colorless ferroelectric‐ferroelastic β′‐Tb₂(MoO₄)₃ (TMO) single crystals have been grown by the Czochralski (CZ) method. The single crystal structure was investigated by X‐ray powder diffraction and was shown to be a single phase with the structure similar as the β′‐Gd₂(MoO₄)₃ crystal. The optical transparency of the TMO crystal has been measured and the crystal is almost transparent in the visible and near infrared regions. The defects of TMO crystal were evaluated by etching technique and the ferroelectric domain structures were observed by an optical microscope. A high‐resolution X‐ray diffraction analysis demonstrates that the as‐grown TMO crystal possesses relatively high optical quality. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)