Hydrothermal synthesis of improved ZnO crystals for epitaxial growth of GaN thin films

ZnO single crystals with thickness up to 12 mm, 2 inches in “diameter” and weight of about 150 g have been grown from KOH, NaOH, and K₂CO₃ based hydrothermal solutions on the seeds of (0001) orientation. The addition of LiOH up to 3.0–4.5 mol/L allowed to decrease the growth rate of ZnO crystals along the 〈0001〉 crystallographic direction. For positive and negative monohedra, it was achieved 0.12 and 0.01 mm/day, respectively, at temperature 340 °С and ΔТ = 10 °С. The best ZnO etching agent was found to be the solutions 25 mol% HCl + 3 mol% NH₄F at room temperature, and etching time 5 min. The dislocation density of ZnO crystals varied from 240 cm⁻² to 3,200 cm⁻² in the case of growth rates 0.04 mm/day to 0.11 mm/day, respectively. It was also found that ZnO crystals grown are stable in air, oxygen, nitrogen, and argon atmosphere as well as in vacuum at the temperatures up to 1,000 °C under thermal treatment during 4 h.     

Growth of Mg2TiO4 single crystals by the floating zone method

Single crystals of the peritectic compound Mg₂TiO₄ up to 8 mm diameter and 80 mm long along the [1 0 0] and [1 1 1] directions, were grown by the travelling solvent float zone (TSFZ) technique using a halogen lamp image furnace. The optimum conditions were as follows: sintering temperature of the charge rod, 1500° C, the growth rate, 2 mm h^–1 and the composition of the charge rod in molar ratio, MgOTiO₂=21.01. The chemical composition of the solvent zone was determined by application of the EPMA technique and the peritectic nature of Mg₂TiO₄ was proved. The refractive index of the grown crystal was 2.05.     

Growth of ZnSiAs2 by vapor transport

We have grown crystals of ZnSiAs₂ by vapor transport in a sealed fused silica container with an argon atmosphere. These vaporgrown crystals have a blade-like habit with the [221] direction perpendicular to the flat surface of the blade. Nominal dimensions are 10 × 1 × 0.1 mm. Buerger precession photographs and powder photographs were used to confirm the crystal structure and composition. ZnAs₂, Zn₃As₂, and As were deposited concurrently in a faceted globule morphology with dimensions up to 4 mm.     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (～300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to ～7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

Ferroelectric phase transition temperatures of self-flux-grown RbTiOPO4 crystals

A series of RTP crystals have been grown by the top-seeded solution growth method from self-fluxes containing different [Rb]/[P] atomic ratios (1.25–2) and solute concentrations (0.55–1.1 g-RTP/g-flux) covering a large portion of the RTP crystallization field in the Rb₂O–P₂O₅–TiO₂ ternary system. The Curie points of as-grown crystals have been determined using the standard dielectric (capacitance) technique, and they show a spread from 770 to 800 °C. This indicates a clear correlation between the crystal stoichiometry and the flux chemical composition in similarity with KTP crystals. The major difference from the KTP case is that each growth sector (solidified simultaneously) in nominally pure RTP crystals exhibits its specific Curie temperature. Chemical analysis and intentional doping experiments show that trace impurities may have a very limited influence on this effect. The abrupt changes of the Curie temperatures along simultaneously grown surfaces are discussed in terms of the diverse incorporation mechanisms of the stoichiometric components or native defects into specific crystallographic faces during growth.     

Cathodoluminescence and scintillation characteristics of YAG:Ce crystals grown by horizontal directional crystallization in a protective atmosphere

YAG:Ce crystals have been grown by a new gas-phase horizontal directional crystallization (HDC) technique in a protective atmosphere and their cathodoluminescence (CL) spectra and scintillation characteristics have been studied. Using this HDC technology, it is possible to obtain large (110 × 150 × 35 mm) crystals with a high specific light yield (15 000–18 000 Ph/MeV) and good amplitude resolution (8–10%) for the excitation with 5.15-MeV α particles from a ²³⁹Pu source. In addition to an intense band at 550 nm due to Ce³⁺ ions, the CL spectra of crystals display an intrinsic emission band of YAG in the UV spectral range, which is due to the presence of Y_Al antisite defects.     

Preparation of single crystals of YBa2Cu3O7−x from nonstoichiometric melts

Studies on the single crystal growth of YBa₂Cu₃O_7−x show that the growth conditions have not been optimised yet and they vary in many ways. Here we report the growth of single crystals of YBCO in the size range 0·5–1·2 mm from nonstoichiometric melts. We have made systematic variations in the flux composition (constituting CuO and BaCO₃) in order to arrive at an optimum composition for consistently getting single crystals of size 0·5–1·2 mm. The tetragonal phase was confirmed by X-ray diffraction and single crystallinity by the Laue technique. Superconductivity was confirmed in oxygenannealed crystals.     

Raman scattering study of molten alkali-metal molybdates and tungstates rich in basic oxides

Structural transformations of alkali-metal (Li, K, and Na) mono- and dimolybdates and tungstates have been studied by Raman scattering spectroscopy from room temperature to above their melting point. The structure of the Mo-O and W-O complexes in the form of isolated [MoO₄] and [WO₄] tetrahedra in crystalline monomolybdates and monotungstates has been shown to persist in the molten state. At the same time, the mixed anion motif made up of tetrahedra and octahedra in dimolybdate and tungstate crystals transforms on melting into complexes made up of two corner-sharing tetrahedra. We have identified the internal vibrational modes of such complexes in molten alkali-metal dimolybdates and ditungstates. Using rapid quenching of K₂O · 2MoO₃ and K₂O · 2WO₃ melts, we have obtained transparent glasses whose Raman spectra suggest that they have the same anion motif as the corresponding melts.     

Structural,optical, electrical properties of new hybrid organic–inorganic NLO single crystal: bis(1H-benzotriazole) hexaaqua-zinc bis(sulfate) tetrahydrate (BZS)

A new hybrid organic–inorganic nonlinear (NLO) single crystal, Bis(1H-benzotriazole) hexaaqua-zinc bis(sulfate) tetrahydrate (BZS), has been successfully synthesized and the single crystals were grown by slow evaporation solution growth technique (SESG) using Millipore water as a solvent. The structure of the BZS crystal was solved and refined by single-crystal X-ray diffraction and demonstrates that the grown crystals belong to a triclinic system with the space group P-1. The asymmetric part of the titled compound contains isolated organic cation (C₆H₆N₃)₂, metallic cation [Zn(H₂O)₆]²⁺, sulfate anion (SO₄)^2? and free H₂O molecules. The interplay between the wide number of intermolecular interaction such as O–H···O, N–H···O, C–H···O and π–π stacking interactions were discussed. The optical transmittance spectrum shows that the crystal is excellent transmittance in the entire Vis–NIR region with the cutoff wavelength at 345 nm. The presences of expected functional groups were identified by Fourier transform infrared spectroscopy. The dielectric measurements were carried out at different temperature in the frequency range 100 Hz–5MHz. Furthermore, the studies of its third-order NLO properties using a Z-scan technique demonstrate that the BZS crystal possesses a strong reverse saturable absorption (RSA) and the self-focusing (SF) nature with large second order hyperpolarizability (γ?=?6.24?×?10^?34 esu). All the results indicate that BZS crystal might be the potential candidate for the third-order NLO applications.     

Vapour phase crystal growth under microgravity environment

Vapour phase crystal growth experiments performed in the Skylab and ASTP missions are reviewed. The basic vapour phase crystal growth technique is described and effect of gravity is discussed. The multipurpose furnace specially designed to carry out various experiments in flight conditions is described. Ge Se, Ge Te and GeS as well as ternary GeSe_0·99 Te_0·01 and GeS_0·98 Se_0·02 crystals have been grown in space showing improvement over similarly grown crystals on ground as determined by x-ray diffraction, chomical homogeneity and surface morphology studies. Mass flux rates under microgravity conditions have been found to be up to 10 times larger than expected indicating need for better theoretical and experimental understanding of the effect of gravity on crystal growth.     

Synthesis, growth, and characterization of nonlinear optical material l-arginine iodate crystal

Single crystals of nonlinear optical l-arginine iodate (l-Arg·2HIO₃) (C₆H₁₄N₄O₂·2HIO₃), were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at a constant temperature (30 °C) from its aqueous solution at pH value of 6. Crystals were of average dimensions 3 × 2.5 × 1.0 cm³ by optimizing the growth parameters. Initially, solubility studies were carried out for four different solvents such as water, water-methanol, water-ethanol, and water-acetone. Among the four solvents, the solubility of l-Arg·2HIO₃ was found to be the highest in water and hence crystallization of l-Arg·2HIO₃ was carried out from aqueous solution. Morphology studies reveal that the crystals grow with reasonable growth rate along the [100], [001], [110¯] and [011] direction. As-grown crystals were characterized by density measurement, X-ray powder diffraction studies, infrared spectrophotometer, UV-Vis spectrometer, laser induced damage threshold studies and nonlinear optical study.     

Single crystal growth in PMN-PT and PMN-PZT

Single crystal growth of lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN-32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN-37PT-21PZ) ceramics via templated grain growth (TGG) was investigated. (001)- and (111)-oriented BaTiO₃ (BT) single crystals and (001)-oriented SrTiO₃ (ST) single crystals (of approximately 2.5 × 2.5 × 1 mm) were utilized as seeds for the growth experiments. The piezoelectric single crystals were produced in a process that involves at first hot pressing of single crystal in cold isostatically pressed ceramics followed by subsequent sintering of the samples. Growth of (001)-oriented single crystals with BT seeds was observed in both PMN-32PT and PMN-37PT-21PZ matrices. The measured growth lengths were up to 140 and 65 μm, respectively. The grown (001)-oriented single crystals grown were rectangular. The measured growth lengths of the pyramidal-shaped (111) BT single crystals were up to 1 mm, which is much larger than the growth lengths of the (001) single crystals. Experiments on (001) ST-seeded single crystals were not successful. No single crystal growth was observed due to the dissolution of the ST single crystals in the PMN-PZT matrix. The differences were explained by defect-chemical considerations.     

Influence of iron and temperature on the crystallization of calcium phosphates at the physiological pH

The effect of iron and temperature on the crystallization of dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) in agarose was studied at various temperatures (27, 37 and 47 °C) at the physiological pH (7.4) by the single diffusion method. The spherulitic crystals of hydroxyapatite with a diameter of about 3 mm were observed in the presence of Fe at temperature of 47 °C. The spherulites consisted of thin, elongated needlelike crystals. Quantification of the iron incorporated in the crystals was carried out by elemental analysis. The crystals were characterized by XRD, FTIR and SEM techniques.     

Crystal growth and structural analysis of zirconium sulphoselenide single crystals

A series of zirconium sulphoselenide (ZrS _x Se_3−x , where x = 0, 0·5, 1, 1·5, 2, 2·5, 3) single crystals have been grown by chemical vapour transport technique using iodine as a transporting agent. The optimum condition for the growth of these crystals is given. The stoichiometry of the grown crystals were confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and the structural characterization was accomplished by X-ray diffraction (XRD) studies. The crystals are found to possess monoclinic structure. The lattice parameters, volume, particle size and X-ray density have been carried out for these crystals. The effect of sulphur proportion on the lattice parameter, unit cell volume and X-ray density in the series of ZrS _x Se_3−x single crystals have been studied and found to decrease in all these parameters with rise in sulphur proportion. The grown crystals were examined under optical zoom microscope for their surface topography study. Hall effect measurements were carried out on grown crystals at room temperature. The negative value of Hall coefficient implies that these crystals are n-type in nature. The conductivity is found to decrease with increase of sulphur content in the ZrS _x Se_3−x series. The electrical resistivity parallel to c-axis as well as perpendicular to c-axis have been carried out in the temperature range 303–423 K. The results obtained are discussed in detail.     

Morphology of solution-grown poly(β-propiolactone) single crystals

Single crystals of poly(-propiolactone) (PPL) with different molecular weights (M _w = 70000 and M _w = 2300) were grown from four kinds of solvents under isothermal crystallization condition. The morphologies and crystal structures of PPL single crystals were investigated by means of transmission electron microscopy and atomic force microscopy. The single crystals of high-molecular-weight PPL (HMW-PPL) grown from cyclohexanone appeared elongated with dimensions of around 0.2–0.8 m along the short and 5–10 m along the long axes. Single crystals of low-molecular-weight PPL grown from cyclohexanone showed three to five elliptical-shaped lamellae, from central nucleus like petals. The long axes of both single crystals corresponded to the crystallographic b axis. The reciprocal lattice parameters: a* = 2.045 nm^–1, b* = 1.420 nm^–1 and * = 90° could be determined from electron diffractograms. Decoration of the crystals with polyethylene suqqested that the single crystals of HMW-PPL have regular chain-folding along their long axis in the [010] direction with consecutive folds in the [110] and [1 0] directions. Accordingly, it is deduced that HMW-PPL has the anti-parallel chain-packing structure.     

Effect of grain size on grain boundary strengthening of copper bicrystals under cyclic loading

Abstract

To clarify the strengthening effect of grain boundaries (GB), cyclic deformation behaviour of really grown [4¯79] ∥ [1¯25] copper bicrystals with different widths (4, 6, and 8 mm, denoted RB-4, RB-6, and RB-8) of com-ponent crystals and a combined copper bicrystal (denoted CB-6), obtained by sticking component single crystals G1 [4¯79] and G2 [1¯25] together, was investigated. The results showed that the cyclic saturation stresses increased in the order of bicrystals of CB-6 < RB-8 < RB-6 < RB-4. It is indicated that the GB effect caused different degrees of strengthening, which increased with the decreasing width of the RB bicrystals. By surface observation, it was found that only the primary slip system was activated in the combined bicrystal during cyclic deformation. However, an additional slip system appeared near the GB within the crystal G2 [1¯25] in the RB bicrystals (except in the primary slip system), and formed a GB affected zone (GBAZ). The width of the GBAZ was about 400 and 600 μm at plastic strain amplitudes of 0·1% and 0·2% respectively. Meanwhile, using an electron channelling contrast (ECC) technique in the SEM, the dislocation patterns near the GB and within the component crystals were observed. It was found that a two phase structure of persistent slip bands (PSBs) and matrix (or veins) can form in these bicrystals, similar to that in copper single crystals. But these PSBs cannot transfer through the GB during cyclic deformation. Based on the results above, the effect of grain size on GB strengthening of copper bicrystals was discussed.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Top-cooling-solution-growth and characterization of piezoelectric 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 [PZNT] single crystals

A technique of top-cooling-solution-growth (TCSG) has been developed to grow the piezo-/ferroelectric perovskite single crystals of 0.955Pb(Zn_1/3Nb_2/3)O₃-0.045PbTiO₃ [PZNT95.5/4.5]. The flux composition and concentration, and the thermal parameters have been optimized, leading to the growth of high quality PZNT crystals with a size up to 20 × 15 × 10 mm³. The perovskite crystals are found to form upon slow cooling down to 1020°C, while the undesirable pyrochlore crystals of Pb_1.5Nb₂O_6.5-type start growing upon further cooling from 1020°C to 950°C. By controlling the growth pathway, the formation of the pyrochlore phase can be avoided. The dielectric properties of the grown PZNT95.5/4.5 crystals have been measured as a function of temperature at various frequencies. Upon heating, the phase transition for the rhombohedral R3m to the tetragonal P4mm phase takes place at 132°C, while the tetragonal to cubic phase transition occurs at 160°C. The TCSG developed in this work provides an alternative technique to grow PZNT piezocrystals of medium size at low cost for transducer applications.     

Thermal Conductivity of Tetragonal Cadmium Diphosphide Crystals

Tetragonal cadmium diphosphide (β-CdP₂) single crystals with high structural perfection have been grown and their thermal conductivity has been measured. The results indicate that the higher thermal conductivity across the c axis corresponds to the smaller unit-cell parameter of the crystals (a = 5.08 ± 0.01 Å and c = 18.59 ± 0.05 Å). The observed thermal conductivity anisotropy is tentatively attributed to the fact that structural basis of β-CdP₂ is made up of spectral phosphorus chains running in the [100] and [010] directions.     

Effect of stoichiometry on the thermal expansion coefficients of lithium niobate single crystals

The chemical analysis of LiNbO₃ single crystals, with different melt compositions (Li/Nb) _m =0·945, 1·0, 1·1 and 1·2, grown by slow cooling technique, reveals a remarkable difference in solid and melt compositions. The thermal expansion coefficients alonga andc axes, determined by using Newton’s ring experiment, are found to increase anisotropically with increase in (Li/Nb) _m ratio. The variation in thermal expansion coefficient with increase in the (Li/Nb) _m ratio is discussed in the light of defect chemistry i.e. the partial replacement of Li⁺ by excess Nb⁵⁺ creates additional cation vacancies to attain the electro-neutrality in the crystal.