Mössbauer studies on some glasses and crystals in the Na2O---Fe2O3---SiO2 system

Mössbauer absorption measurements have been made at room temperature on ⁵⁷Fe in iron sodium silicate glasses containing 3–15 mol% Fe₂O₃ and various iron alkali silicate crystals in order to study the state of iron in these glasses. The spectra of all the glasses gave one doublet with a quadrupole splitting varying from 0.73–0.78 mm s⁻¹, while those of Na₂O · Fe₂O₃ · 4 SiO₂ and 5 Na₂O · Fe₂O₃ · 8 SiO₂ crystals showed much smaller quadrupole splitting, 0.28 mm s⁻¹ and 0.10 mm s⁻¹, respectively, and an asymmetrical doublet of much narrower linewidth. When sodium was replaced by other alkali metals of larger size, such as K and Cs, in MFeSi₂O₆ and MFeSi₃O₈ crystals, the quadrupole splitting became wider and approached to 0.73 mm s⁻¹. Such a variation was not observed for glasses. These results suggest that a larger number of non-identical sites exist in iron sodium silicate glasses than in the corresponding crystals.     

Growth of Na modified potassium lithium niobate crystal by the Czochralski method

Transparent Na modified potassium lithium niobate (Na_0.23K_2.60Li_1.82Nb_5.35O_15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na₂O:K₂O : Li₂O:Nb₂O₅=2:30:25:43 mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.5446±0.0010 Å and c=4.0129±0.0005 Å at room temperature. The dielectric constant along the c-axis ε₃₃ showed a sharp maximum around 480 °C. Optical transmission edge was 370 nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800 nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal.     

A new technique for the growth of superconducting YBa2Cu3O6 + δ crystals completely separated from flux

Growth of completely flux-separated YBa₂Cu₃O_{6 + δ} (referred to as 123 phase) crystals using a novel technique is described. The technique employs a modification of the seed pulling method commonly used in crystal growth. The crystals are grown in the temperature range of 960–1000°C using a BaCuO₂ flux. A 123 flux ratio of 1:5 is maintained. Photographs of the crystals and photomicrograph of the surfaces are presented to show complete flux-separation of the crystals measuring 6 mm × 3 mm × 1 mm. The Raman spectra recorded on the as-grown crystals show that they are in the tetragonal phase. Magnetic susceptibility measurements on crystals annealed in an oxygen atmosphere show a superconducting transition starting at 71 K. The present technique offers a possibility of growing large, completely flux-separated crystals of 123 for superconductivity research.     

Crystallization of In2O3 by vapor reaction

Crystallization of In₂O₃ occured in closed porcelain crucibles in air at 960–1200°C by vapor phase reaction of In₂O or In vapor with the oxygen diffusing into the system. The In₂O or In vapors were thermally generated from mixtures such as graphite/In₂O₃, graphite/In, In₂O₃/In and graphite/In₂O₃/In. The graphite/In₂O₃ system at a mole ratio of 30/1 and 1000°C produced yellow, transparent needle crystals with a maximum size of 0.5 X 0.5 X 8 mm and electrical resistivity of 5.5 X 10^-2 ω cm at 25°C.     

Hydrolytic resistance of Na2O---B2O3---SiO2 gels prepared by sol-gel process

Alkoxide derived gels were prepared in the system Na₂O---B₂O₃---SiO₂. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram.

Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na₂O, B₂O₃ and SiO₂ extracted from the attack gels were analyzed. The experimental results indicate that the amount of B₂O₃ has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B₂O₃ mol% the greater are the amounts of Na₂O and B₂O₃ extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B₂O₃ and Na₂O extracted slightly increases as the B₂O₃ mol% increases. Small amounts of extracted SiO₂ were always observed.

These results are complemented with other measurements so that an explanation of the controlling mechanism is given.     

Research on growth and self-frequency doubling OF Nd:ReCOB (Re=Y OR Gd) crystals

Large (up to 45mm in diameter and 50mm in length) ReCa₄O(BO₃)₃ (ReCOB) and Nd:ReCOB (Re=Y or Gd) crystals with high optical quality were grown by well designed procedures. Crystal orientation was carefully determined. Based on the measurements of refractive indices the phase matching (PM) figures for both crystals were calculated. Theoretical calculation of the spatial distribution of the effective nonlinear coefficients, which show an asymmetric feature, was carried out to determine the optimal PM orientations which were experimentally confirmed, i.e. (66.3°, 134.4°) and (64.5°, 33.5°) for GdCOB and YCOB respectively.     

The effect of MgO on the thermodynamics of the Fe/Fe-redox equilibrium and the incorporation of iron in soda-magnesia-aluminosilicate melts

Melts with the basic compositions 10Na₂O · 10MgO · xAl₂O₃ · (80−x)SiO₂ (x=0, 5, 10, 15 and 20), 10Na₂O · xMgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10, 15 and 20) and xNa₂O · 10MgO · 10Al₂O₃ · (80−x)SiO₂ (x=5, 10 and 15) all doped with 0.25 mol% Fe₂O₃ were studied using square-wave voltammetry. The temperatures applied were in the range of 1000–1600 °C. The square-wave voltammograms recorded show peaks caused by the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials measured decreased linearly with decreasing temperatures. Increasing the MgO-concentration led to more negative peak potentials. Introducing alumina in the melt first resulted in less negative peak potentials. If the molar Al₂O₃-concentration is equal to that of Na₂O (=10 mol%) the peak potentials are least negative. Further increase of the Al₂O₃-concentration led to more negative peak potentials. The variation of the Na₂O-concentration led to a maximum in the peak potentials at an Na₂O-concentration of 10 mol%. An empirical formula which allows the calculation of standard potentials from the chemical composition is proposed. Furthermore, a structural explanation for the effect of the chemical composition is given. Especially, the incorporation of Al₂O₃ as AlO₄⁻-tetrahedra at [Al₂O₃] < [Na₂O] and as network modifier at larger concentrations was structurally explained by the similarities of Fe²⁺ and Mg²⁺, with respect to cation radii and metal–oxygen bond lengths.     

Experimental observation of the surface roughening transition in vapor phase growth

The surface roughening transition has been observed in growth from the vapor phase in both C₂Cl₆ and NH₄Cl. Both these materials are plastic crystals which have cubic structures and sublime at atmospheric pressure rather than melting. The surface roughening transition was detected by observing changes in the growth morphology in these two materials. In C₂Cl₆ the roughening transition is approximately at 100°C where the equilibrium vapor pressure is 40 mm, and in NH₄Cl it is at approximately 365°C and 3 atm pressure.     

Growth and properties of Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals directly from melt

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by a modified Bridgman method directly from melt using an allomeric Pb[(Mg_1/3Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal as a seed. X-ray diffraction (XRD) measurement confirmed that the as-grown PZNT91/9 single crystals are of pure perovskite structure. Electrical properties and thermal stabilization of PZNT91/9 crystals grown directly from melt exhibit different characters from those of PZNT91/9 crystals grown from flux, although segregation and the variation of chemical composition are not seriously confirmed by X-ray fluorescence analysis (XPS). The [0 0 1]-oriented PZNT91/9 crystals cut from the middle part of the as-grown crystal boules exhibit broad dielectric-response peaks at around 105 °C, accompanied by apparent frequency dispersion. The values of piezoelectric constant d₃₃, remnant polarization P_r, and induced strain are about 1800–2200 pC/N, 38.8 μC/cm², and 0.3%, respectively, indicating that the quality of PZNT crystals grown directly from melt can be comparable to those of PZNT91/9 single crystals grown from flux. However, further work deserves attention to improve the dielectric properties of PZNT crystals grown directly from melt. Such unusual characterizations of dielectric properties of PZNT crystals grown directly from melt are considered as correlating with defects, microinhomogeneities, and polar regions.     

Top-seeded solution growth of LiB3O5

We report on the top-seeded solution growth of LiB₃O₅ from an excess B₂O₃ solution. Parameters investigated include the Li₂O/B₂O₃ ratio, rotation rate, pulling rate, cooling rate, and seed direction. With careful control of the above parameters, we have grown clear crystals of 25 × 30 × 20 mm in size. Selected nonlinear optical properties of these crystals are reported. Observations concerning the occurrence of unstable growth (inclusions, hopper growth) are discussed, and methods to eliminate the unstable growth are suggested.     

Properties and structure of RO---Na2O---Al2O3---P2O5 (R = Mg, Ca, Sr, Ba) glasses

The properties and structure of (45 - x)RO · xNa₂O · 2.5Al₂O₃ · 52.5P₂O₅ (R = Mg, Ca, Sr, Ba, 0 x 31 mol%) glasses were investigated. The variation in the molar volumes of glasses in the MgO series is closely related to the formation of the end groups in the glasses with the substitution of Na⁺ ions for Mg²⁺ ions, resulting in a variation of the density and refractive index of the glasses. The properties of glasses containing CaO in terms of Na₂O substitution depend mainly on the low field strength of Na⁺ ions substituting for CaO even though the end groups occurring in the glasses increased. The variation in properties of the glasses containing SrO and BaO, some of which were substituted by Na₂O, could be explained by differences in masses, field strength and polarizability between the Na⁺ ions and the alkaline-earth ions due to a small variation in the structure of the glasses despite Na₂O substitution.     

An X-ray diffraction study of the structure of B2O3---Cs2O liquids

The short range structures of B₂O₃ (90 mol%)---Cs₂O (10 mol%) and B₂O₃ (80 mol%)---Cs₂O (20 mol%) liquids were analyzed at 973 and 1053 K, respectively, by an X-ray diffraction method, and the effects of Cs₂O addition on the boron-oxygen bonding were investigated. The existence of BO₃ triangles, which form the so-called boroxol ring structure, was confirmed in B₂O₃---Cs₂O liquids, as well as in B₂O₃ liquid, but some fraction of the BO₃ triangles was thought to be converted to BO₄ tetrahedra. Similar results have previously been observed also in B₂O₃---Cs₂O glasses. A Cs atom was found to be surrounded by six O atoms; four Cs---O interatomic distances were about 3.2 Å but the other two were at 3.8–3.9 Å. These distances indicate that distorted Cs---O octahedra may exist in these B₂O₃---Cs₂O liquids.     

Li conductivity in lithium niobate: silica glasses

Glasses of composition 65 mol% LiNbO₃:: 35 mol% SiO₂ have been shown to be Li⁺ ion conductors with a conductivity at 200°C > 1 × 10⁻⁵ (η cm)⁻¹ and an activation energy of 0.54 eV. The addition of approximately 0.1 mol% Fe₂O₃ leads to an enhancement of conductivity to ≈10⁻³ (η cm)⁻¹ at 200°C and an activation energy of 0.67 eV. The effect of Fe is shown to be in the control of microstructure in the glass, with Fe₂O₃ concentrations < 1 mol% acting as a grain growth inhibitors and larger concentrations acting as a nucleating agents. A model for this process based on the expected stoichiometry of the melt and the effect of Fe²⁺ and Fe³⁺ in charge compensation is in excellent agreement with experimental data from electron spin resonance.     

Growth and morphology of ruby crystals with unusual chromium concentration

Single crystals of ruby have been obtained from fluxed melts based on the systems Li₂O–MoO₃, Li₂O–WO₃, Na₂O–WO₃, 2PbO–3V₂O₅, PbO–V₂O₅–WO₃, PbF₂–Bi₂O₃ and Na₃AlF₆ by both the TSSG method and spontaneous crystallization at the temperatures 1330–900 °C. Al₂O₃ solubility has been measured for the flux composition of 2Bi₂O₃–5PbF₂ in the temperature range 1200–1000 °C and dissolution enthalpy has been defined as 29.4 KJ/Mol. The composition of grown crystals was studied by electron microprobe analysis. The synthetic ruby contains from 0.51 to 6.38 at% of chromium admixture depending on the crystal growth conditions. Experimental results on growth conditions, composition and morphology of grown crystals are presented for each flux and temperature interval.     

Amorphous phase separation and crystallization in a lithium silicate glass prepared by the sol-gel method

A 10Li₂O---90SiO₂ (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li₂O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.     

Single-crystal growth of sulphospinel CuIr2S4 from Bi solution

Single crystals of a sulphospinel CuIr₂S₄ have been grown from bismuth solution by a slow cooling method for the first time. The grown crystals have a maximum edge of about 1 mm in size and a mirror-like shining surface. Optimum growth conditions are fairly strict. The specific weight of starting materials for the crystal growth is found to be 0.30 g of CuIr₂S₄ and 10.0 g of Bi in order to obtain good quality crystals. The starting and finishing temperatures for the slow cooling step in the temperature control are 1000 and 500°C. The pertinent cooling rate is 2°C/h. Since the volume of bismuth itself expands in the transition from liquid phase to solid phase, a simple method of separation of the grown crystals from the liquid solution will be proposed for avoiding the mechanical damages to the grown crystals. The single crystals have the normal-spinel structure of the lattice constant a=9.849 Å at room temperature. A step-like anomaly in the susceptibility of the single crystals, corresponding to the metal–insulator transition in the resistivity, occurs much sharply than in the powder specimen.     

Calculation of NMR shieldings and other properties for three and five coordinate Si, three coordinate O and some siloxane and boroxol ring compounds

Valuable information on the geometric and electronic structure of both predominant and defect species in the bulk and on the surface of amorphous SiO₂ can be obtained from solid state ²⁹Si and ¹⁷O NMR. Most previous NMR.studies have made assignments of structure types to NMR signals by relying on a comparison to crystalline reference materials. Using the quantum mechanical technique of Coupled Hartree-Fock Perturbation Theory (CHFPT) we can directly calculate NMR shielding tensors, σ, for Si and O and electric field gradients tensors, q for O in silicates. Previous CHFPT calculations on (SiH₃)₂O at Si---O---Si = 180° to 140° semiquantitatively reproduced observed trends in σ for SiO₂ polymorphs. These calculations have been improved and extended to Si---O---Si = 120° in (SiH₃)₂O and to the small siloxane rings H₄Si₂O₂, H₆Si₃O₃, H₈,Si₄O₄ which are models for the ring structures which may occur in the bulk and surface of amorphous SiO₂. Thus, predictions can be made of the ¹⁷O and ²⁹Si NMR characteristic of such species. We have also calculated NMR shieldings and other properties for three and five coordinate Si and for three coordinate O. The ²⁹Si NMR shielding increases with coordination, as expected, and five coordinate Si shows an anisotropy which although large is still smaller than that inferred from spin-lattice relaxation times in silicate melts. O q values are also calculated for (BH₂)₂O and H₃B₃O₆. The results are consistent with O EFGs and Raman frequencies observed in B₂O₃ glass, supporting a boroxol ring model.     

Electrical conductivities of mixed cation glasses

The electrical conductivities of (1−x) Li₂O · x BaO · 2 SiO₂, (1−x) Na₂O · x MgO ·2 SiO₂, (1−x) Na₂O · x CaO · SiO₂ and (1−x) Na₂O · x BaO · 2SiO₂ glasses were measured at temperature ranging from room temperature to 450°C. The transport numbers for Na⁺ ion in (1−x) Na₂O · x BaO · 2 SiO₂ glasses were measured. It was found that the alkali ion carried a significant part of the current in these glasses except one that had no alkali ions, and the conductivity decreased markedly as the alkali oxide was substituted by an alkaline earth oxide. The results of conductivity measurements combined with the data hitherto reported on mixed alkali glasses led to the proposal that the so-called mixed alkali effect could be explained on the basis of the independent path model in which it is assumed that cations can move only through vacant sites left by those of the same type.     

Characteristic crystal growth from amorphous WO3 film by vacuum heating

Selective growth of WO₂, W and WO_3−x crystals from amorphous WO₃ film by vacuum heating at 400–900°C was clarified. The grown WO_3−x crystals were incommensurate structure based on crystallographic share structure. The growth process of WO₂ crystal in the amorphous film was directly observed at high temperature in the electron microscope. The growth front of the WO₂ crystal consumes WO₃ microcrystallites with various orientations. The growth speed of the WO₂ depended on WO₃ microcrystallites orientation. The origin of the wavy growth front of WO₂ was due to an orientation dependence of the WO₃ microcrystallites.     

Anomalous effect of temperature on atomic layer deposition of titanium dioxide

A strong influence of growth temperature on the deposition rate and refractive index of TiO₂ thin films, grown by atomic layer deposition from TiCl₄ and H₂O, was observed. The growth rate increased from 0.05 to 0.09 nm per cycle while the refractive index decreased from 2.63 to 2.00 with the increase of growth temperature from 150°C to 225°C. The effect was due to crystallization processes starting at these temperatures. The substrate temperature range, in which the growth rate most significantly changed, depended on the TiCl₄ pulse time.