Investigation of the structure and properties of quartz in the α-β transition range by neutron diffraction and mechanical spectroscopy

The paper reports on the results of complex investigations into the physical properties of synthetic quartz single crystals and quartz powders in the temperature range of the α-β transition with the use of neutron diffraction and mechanical spectroscopy. The crystal structure of quartz powders with different average sizes of grains is determined in the temperature range up to 620°C and in the α-β transition temperature range. The temperature dependences of the internal friction and the resonant frequency for quartz samples in the vicinity of the phase transition temperature are obtained upon excitation of vibrations in the planes parallel and perpendicular to the Z axis of the quartz crystal. The temperatures at the maxima of the internal friction in the range 560–620°C are determined. The assumptions regarding the possible reasons for the shift of the phase transition temperature are made. It is revealed that the internal friction is characterized by a maximum that is observed in the vicinity of 350°C and is not related to the structural transformations in quartz.     

The dependence of cobalt concentration on the growth temperature in synthetic blue quartz

Co-doped blue quartz crystals were grown hydrothermally in an Na₂CO₃ solution. The concentration of cobalt in as-grown quartz crystal was observed to be independent of the concentration of cobalt in the nutrient but to be strongly affected by the growth temperature. It is concluded that transparent dark-blue quartz could be grown in the narrow temperature range, that is, 340–345°C. From TEM observations, the origin of the blue color in synthetic blue quartz is thought to be an inclusion effect resulting from the entrapped clusters of Co–Si–OH colloids, which is different from other reports [Wood, Ballman, Am. Mineral. 51 (1966) 216; Lehmann, J. Phys. Chem. Solids 30 (1969) 395]. The as-grown synthetic blue quartz crystal was jewel grade.     

Growth and dielectric properties of Ta2O5 single crystal by the floating zone method

Large Ta₂O₅ single crystal with high‐dielectric permittivity was successfully grown by floating zone (FZ) method under air atmosphere. The grown crystal that has been obtained was typically about 8 mm in diameter and 90 mm in length. The crystal growth parameters were optimized. The crystal symmetry, characterized by means of X‐ray diffraction (XRD), was found to be tetragonal. The relative permittivity and loss tangent along growth and [001] direction were measured in the temperature range between ‐200 °C and 200 °C, which showed a strong dielectric anisotropy. At a frequency of 1 MHz and 20 °C, the dielectric permittivity along the growth direction and [001] direction are 81.17 and 25.04 respectively. The stabilization of high‐temperature phase can explain the dielectric enhancement.     

Synthesis,growth and characterization of a new nonlinear optical crystal: l‐arginine maleate dihydrate

A new nonlinear optical material L‐arginine maleate dihydrate, C₆H₁₄N₄O₂,C₄H₄O₄,2H₂O (LAMD) was synthesized and single crystals were grown by slow cooling and also by slow evaporation method at constant temperature from its aqueous solution. Quality and size of the crystals are found to be dependent on pH of the solution and best crystals were obtained at pH = 4. Single crystal X‐ray diffraction analysis reveal that the crystal lattice of LAMD is triclinic with unit cell parameters a = 5.264(3)Å, b = 8.039(3)Å, c = 9.784(3)Å, α = 106.19(3)°, β = 97.24(3)°, γ = 101.66(2)°. Second harmonic generation efficiency is found to be about 6.8 times that of quartz. It is optically transparent down to 300 nm and possesses a large optical window between 300–2000 nm. The compound is thermally stable up to 93.4 °C. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nucleation in quiet supersaturated ADP solutions

The nucleation study has been made with supersaturated ADP solution. Rate of nucleation as indicated by the induction period was measured for non-agitated system over the temperature range 20°C–45°C. The nucleation rate increases with increase in temperature but supersaturation has the dominant effect as predicted by classical nucleation theory. However, attempts to analyse the results in accordance with classical theory were not entirely successful, but it is shown how the assumption of (a) a variation of crystal surface energy with temperature and (b) the influence of heterogeneous nucleation can account for the discrepancies. Activation energies, surface free energies and sizes of crystal nuclei were determined for nuclei of ammonium dihydrogen orthophosphate over the range of temperature 20°C–45°C.     

Synthesis of nano‐crystalline zeolite β: Effects of crystallization parameters

The effects of variation in Si/Al ratio (25 and 100) and crystallization temperature (80 °C to 180 °C, at an interval of 20 K) on crystal size of zeolite β were studied. Products obtained at different synthesis parameters were characterized by powder X‐ray diffraction, IR spectroscopy, thermal analysis, scanning electron microscopy and nitrogen adsorption. Increase in crystal size with crystallization temperature and Si/Al molar ratio was observed. Crystal morphology at 140 °C was spherical whereas at 180 °C it was of irregular shape. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron microscopy of biomaterials based on hydroxyapatite

Three types of biomaterials based on hydroxyapatite are synthesized and investigated. Hydroxyapatite nanocrystals or microcrystals precipitated from low-temperature aqueous solutions serve as the initial material used for preparing spherical porous granules approximately 300–500 μm in diameter. Sintering of hydroxyapatite crystals at a temperature of 870°C for 2 h or at 1000°C (for 3 h) + 1200°C (for 2 h) brings about the formation of solid ceramics with different internal structures. According to the electron microscopic data, the ceramic material prepared at 870°C is formed by agglomerated hydroxyapatite nanocrystals, whereas the ceramics sintered at 1200°C (with a bending strength of the order of 100 MPa) are composed of crystal blocks as large as 2 μm. It is established that all the biomaterials have a single-phase composition and consist of the hydroxyapatite with a structure retained up to a temperature of 1200°C.     

Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

A new piezoelectric single crystal obtained by Ge doping in the SiO2 structure

The interest of Si_1–xGe_xO₂ single crystals with alpha‐quartz structure is connected to improvement of electromechanical coefficients and rise of α – β phase transition of quartz one. Growth of an α‐Si_xGe_1–xO₂ crystal was realized by a hydrothermal method of temperature gradient in autoclaves, made from Cr–Ni alloys. Nutrient material was prepared from synthetic quartz as crashed rods and placed in the bottom of autoclaves. There was loaded GeO₂ powder additive in proportions to quartz nutrient. Single crystals were investigated by electron microprobe analysis, X‐ray diffraction and atomic force microscopy. The most important result, which was obtained during the investigations, is an experimental proof of growth of α‐Si_xGe_1–xO₂ single crystals under the hydrothermal conditions. The present results thus open the possibility to tune the piezoelectric properties of these materials by varying the chemical composition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐temperature infrared and dielectric properties of large sapphire crystal for seeker dome application

In this paper, large‐sized (∅︁230 mm × 210 mm, 27.5 kg) sapphire was successfully grown by SAPMAC (sapphire growth technique with micro‐pulling and shoulder‐expanding at cooled center) method; and hemisphere dome (140 mm diameter, 5 mm thickness) was fabricated from as‐grown boule. Also, its high temperature infrared transmission (2∼7 µm, 20–800°C) and microwave dielectric properties (8–16.5 GHz, 30–1300°C) were investigated. The experimental results show that sapphire crystal exhibits high infrared transmittance (3 μm, 80–86%), essentially negligible dielectric loss (4.9×10^‐5–3.7×10^‐4), but fairly high dielectric constants (ε=9.4–12.5) in the temperature range of 30–1300°C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,structural and thermal studies of tetrathioureacopper(I) chloride crystals

Tetrathioureacopper(I) chloride, hereafter abbreviated as TCC, was synthesised and single crystals were obtained from saturated aqueous solution by slow evaporation (solution growth) method at room temperature. The crystals obtained are bright, colourless and transparent having well defined external faces. The grown crystals were characterized through elemental analysis, single crystal X‐ray diffraction study, thermal analysis, electron spin resonance spectroscopy and Fourier Transform infrared spectroscopy. The elemental analysis confirms the stoichiometry of the compound. The single crystal diffraction studies indicate that TCC crystallises in the tetragonal lattice and the unit cell parameters are a = b = 13.4082 Å, c = 13.8074 Å, V = 2482.29 ų, α = β = γ = 90°. Space group and the number of molecules per unit cell (Z) are found to be P4₁2₁2 and 8 respectively. The TG curve of the sample shows a prolonged decomposition from 210 to 628.3 °C, from which the decomposition pattern has been formulated. The endothermic peaks in the DTA curve indicate melting and decomposition of the compound at 165.2 and 633.8 °C respectively. An exothermic peak in high temperature DSC indicates a phase transition in the compound at 274.8 °C. Thermal anomalies observed in the low temperature DSC at –163.3, –152.0, –141.5, –108.3, 1.0 and 12.1 °C in the heating run and –157.1 and –153.9 °C in the cooling run reveal first order phase transitions in the crystal. The peaks observed at –146.2 °C in both the heating and cooling runs suggest occurrence of a second order phase transition in this compound. The IR spectroscopic data were used to assign the characteristic vibrational frequencies of various groups present in the compound. The ESR study confirms that the copper is in the +1 oxidation state in the complex. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of ZnO nanorods by the thermal reduction process of a mixture of ZnO and Al powder

Single‐crystalline Zinc oxide (ZnO) nanorods were firstly synthesized on gold‐coated Si substrate via a simple thermal reduction method from the mixture of ZnO and Al powder. The growth process was carried out in a quartz tube at different temperature (550‐700 °C) and at different oxygen partial pressure. Their structure properties were investigated by X‐ray diffraction (XRD), scanning electron microscope (SEM), X‐ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The length of the as‐prepared ZnO nanorods was up to several micrometers and their diameters were about 130 nm. The X‐ray diffraction patterns, transmission electron microscopic images, and selective area electron diffraction patterns indicate that the one‐dimensional ZnO nanorods are a pure Single‐crystal and preferentially oriented in the [0001] direction. The reaction mechanism of ZnO nanorods was proposed on the basis of experimental data. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of sulphamic acid single crystals grown by Sanakaranarayanan‐Ramasamy (SR) method

By directional solidification, single crystal of Sulphamic acid (SA) was successfully grown from aqueous solution by Sankaranarayanan‐Ramasamy (SR) method. A vertically designed L‐bend was used to avoid the effect due to spurious nucleation. A vertical bottom‐seeded ampoule was used for the growth of single crystal. A seed crystal was mounted at the bottom of the ampoule. Sulphamic acid crystals of up to 40 mm in diameter and 60 mm in length have been grown with a growth up to 10 mm per day. The grown sulphamic acid single crystal was characterized using X‐ray powder diffraction analysis, Raman, FTIR, and optical transmission studies. The dielectric behaviour was measured in the frequency range of 1 kHz–10 MHz for the temperature ranges from 30 °C to 170 °C. The sulphamic acid single crystal was also grown by conventional method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Epitaxy of CdTe on sapphire substrates with titanium buffer layers

The formation of a developed electrical relief on the sapphire substrate surface is investigated. A technique is proposed for introducing Ti⁴⁺ impurity atoms into the sapphire crystal lattice by depositing titanium layers with a thickness of about 5 nm and their annealing in air (oxidizing atmosphere) to a temperature of 1400°C. It is shown that this preliminary treatment of the sapphire substrate surface results in epitaxial growth of (111) СdTe films parallel to the sapphire (0001) plane at a temperature of 350°C.     

Measurement of specific surface free energy of ruby and quartz single crystals using contact angle of liquids

The specific surface free energy of ruby and quartz single crystal was experimentally obtained using contact angle of water and formamide droplets on the crystal surfaces, and compared with the morphology of each crystal. The ruby crystals satisfied Wulff's relationship even though their shape were not equilibrium form. The specific surface free energies of the growing faces of synthetic quartz crystal, ‐X, +X, Z, and S faces were obtained as 51.9, 55.6, 57.4, and 58.9 mN/m, respectively. The growth rates of these faces were 0.09, 0.23, 0.28, and 0.33 mm/day, respectively. The growth rate of each face of the quartz crystal can be regarded as a function of the experimentally obtained specific surface free energy. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The study of removing hydroxyl from silica glass

The removal of hydroxyl from silica glass produced by melting quartz powder under an atmosphere containing hydrogen was investigated. After heat-treatment at the temperature range (700–1200 °C) in nitrogen atmosphere, the effective hydrogen diffusion coefficients were evaluated based on the law of nonsteady-state diffusion. The activation energy obtained is 254 kJ mol⁻¹ for the dehydroxylation process in the heat-treatment temperature range of 700–900 °C, and a different activation energy calculated is 32 kJ mol⁻¹ in the temperature range of 900–1200 °C. The activation energies for the dehydroxylation process at the temperature (700–900 °C) and the higher temperature (900–1200 °C) correspond to the binding energy of SiO–H bond and the activation energy for the diffusion of hydrogen in silica glass respectively, which indicate there is a change of mechanism for dehydroxylation with heat-treatment temperature.     

Nucleation kinetics and growth aspects of organic nonlinear optical L‐arginine trifluoroacetate single crystals

Experimental determination of solubility, metastable zonewidth and induction period for an organic nonlinear optical (NLO) L‐arginine trifluoroacetate (LATF) crystal is reported. The interfacial tension for different supersaturation conditions has been estimated for the LATF solution at 45°C. Bulk crystal of dimensions 57x5x3 mm³ has been grown by temperature lowering technique using optimized growth parameters. Powder X‐ray diffraction and FTIR studies confirm the crystalline nature and the functional groups of the grown crystals respectively. Linear optical property of the grown crystal has been studied by UV‐Vis spectrum. Thermal analyses have revealed that the compound is thermally stable upto 212°C. Kurtz powder SHG test confirm the nonlinear optical properties of the as‐grown LATF crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of the vanadium pentoxide V<Subscript>2</Subscript>O<Subscript>5</Subscript> crystal surface after vacuum annealing

A complex study of the (001) cleavage surface of a V₂O₅ single crystal annealed in vacuum at 450 and 550°C has been performed. Tunnel microscopy of the sample surface annealed in vacuum at 550°C showed the formation of a plane with a corundum structure, reconstructed according to the V₂O₃ (0001)-(1/√3 × 1/√3)R30° type, on the surface. X-ray photoelectron spectroscopy revealed a significant modification of interatomic bonds in the surface layers of V₂O₅ single crystal after vacuum annealing at 550°C, which is related to the partial reduction of V⁵⁺ ions and formation of lower vanadium oxides. These modifications lead to a decrease in the electrical resistivity of V₂O₅ and the occurrence of bending at a temperature of 61.5°С in the temperature dependence of the sample resistance, which is indicative of phase transition.     

Local selective homoepitaxy of silicon at reduced temperatures using a silicon-iodine transport system

Selective deposition of homoepitaxial silicon can be achieved using silicon-iodine transport within a quartz ampoule at temperatures down to nearly 600°C. A segregation of gaseous components due to their density differences and a temperature gradient along the deposition region serve to optimize the silicon deposition. The variation of the epitaxial growth process with total pressure and temperature has also been studied. The process shows basic advantages for silicon device applications.     

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)