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Phase transitions and microwave dielectric properties in the (1-x)Ca(Al[sub 0.5]Nb[sub 0.5])O[sub 3]–xCaTiO[sub 3] system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al[sub 0.5]Nb[sub 0.5])O[sub 3], tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO[sub 3] and Ca(Al[sub 0.5]Nb[sub 0.5])O[sub 3] are described by orthorhombic Pbnm and monoclinic P2[sub 1]/n symmetries, respectively, with similar lattice parameters, &sqrt;2a[sub c]×&sqrt;2a[sub c]×2a[sub c] (where a[sub c] is the lattice parameter of cubic perovskite). The (1-x)Ca(Al[sub 0.5]Nb[sub 0.5])O[sub 3]–xCaTiO[sub 3] system features both cation ordering and octahedral tilting phase transitions. The Ca(Al[sub 0.5]Nb[sub 0.5])O[sub 3] structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with increasing Ti content, which correlates with a progressive increase of cation disorder in the specimens. A disordered structure is attained at x=0.5. For the “solid solutions,” the nonlinear dependence of both permittivity ε and the temperature coefficient of the resonant frequency τ[sub f] on Ti content corresponds to a linear dependence of the macroscopic polarizability on composition; that is, the oxide additivity rule was closely obeyed. Therefore, this rule can be used to predict ε and τ[sub f] for any intermediate composition from the permittivities and temperature coefficients of permittivity of the end compounds. A zero temperature coefficient of the resonant frequency occurs at the composition xapprox. 0.5 with a relative permittivit... [ABSTRACT FROM AUTHOR]

Deals with a study which demonstrated the deposition of thin films of the high-temperature superconductor Nd[sub1.85]Ce[sub0.15]CuO[sub4-y] on MgO and SrTiO[sub3] substrates. Background on high-temperature superconductors; Apparatus used for pulsed laser deposition; Optimization of the superconducting properties.

Hollandite solid solutions along the A-site compositional range from the pure barium end-member Ba1.33Ga2.66Ti5.34O16 to the pure cesium end-member Cs1.33Ga1.33Ti6.67O16 have been synthesized using a solid-state reaction technique. The crystal structure of the hollandite across the entire compositional range remained in the I4 /m space group. Structural evolution was resolved by neutron diffraction, total scattering data, and density functional theory calculations. A trend of decreasing thermodynamic stability with smaller tunnel cations was attributed to increased structural distortion observed in the system. In addition, the tunnel cations' local environment was studied in the eightfold coordinated oxygen cavities. Local binding features of the tunnel cations reveals that the hollandite structure can strongly stabilize tunnel cations, even at elevated temperatures up to 500 K. [ABSTRACT FROM AUTHOR]

The thermal expansion of Dy2TiO5 in the hexagonal phase was evaluated and compared with the orthorhombic phase using in situ high-temperature X-ray diffraction. The crystal structure, volume changes before and after the transformation process, as well as the mechanism behind the thermal expansion behavior was determined and proposed. It was found that in the hexagonal phase, the thermal expansion was caused by the oxygen anions in the axial positions of the trigonal bipyramidal structure moving toward the central Ti atom. While expanding, the movement of these oxygen anions slows the expansion along the c-axis resulting in a decrease in α33 with temperature. Furthermore, a structural relationship between the orthorhombic and the hexagonal phases was proposed. [ABSTRACT FROM AUTHOR]

Luminescent fine crystals of Y1.00− xEu xTiNbO6, x = 0-1.00 with high crystallinity were directly synthesized by mild hydrothermal method from weakly basic precursor solution mixtures of YCl3, EuCl3, TiOSO4, and NbCl5. Orthorhombic aeschynite-type crystals in the range of 0.5-2.0 μm consisting of crystallites with 33-91 nm based on a complete solid solution in the YTiNbO6-EuTiNbO6 system were hydrothermally formed at 240°C for 5 h. A single phase of as-prepared aeschynite-type structure was maintained in all the (Y,Eu)TiNbO6 solid solution after heating at temperatures up to 1000°C for 1 h in air. In the range of composition x ≤ 0.6, the as-prepared aeschynite-type (Y,Eu)TiNbO6 solid solutions transformed to a single phase of euxenite structure after heat treatment at temperatures higher than 1100°C-1200°C. The as-prepared (Y,Eu)TiNbO6 fine crystals in the range of composition x = 0.75-0.90 showed the strongest luminescence in the red spectral region: strong red (5D0→7F2 transition of Eu3+) and weak orange light (5D0→7F1) line spectra among the as-prepared and heat-treated samples. Another wet chemical synthesis route confirmed the advantage in directly synthesizing the (Y,Eu)TiNbO6 crystals through this hydrothermal method because heating at 1200°C for 1 h in air was necessary for obtaining crystalline (Y,Eu)TiNbO6 with sufficient luminescence intensity in a composition Y0.70Eu0.30TiNbO6 from amorphous powders that were formed via co-precipitation method. [ABSTRACT FROM AUTHOR]

The compounds formed from the Lu2O3-Ta2O5 system in the composition range 25-60 mol% Ta2O5 were prepared by solid-state reaction from 1350°C to 2058°C, and the phase transitions were investigated by X-ray diffraction (XRD). Cubic Lu3TaO7, M′-LuTaO4, M-LuTaO4, O-Ta2O5, and T-Ta2O5 are observed. With the temperature increase, there is an irreversible phase transition from M′ to M-LuTaO4 near 1770°C in the composition of 30-52 mol% Ta2O5, and another phase transition from T-Ta2O5 to O-Ta2O5 at about 1685°C when the ratio of Ta2O5 is >52 and ≤60 mol%. A phase diagram of the Lu2O3-Ta2O5 system in the range 0-100 mol% Ta2O5 was constructed. These results are helpful to explain the phase transition of Lu2O3-Ta2O5 system and design the preparation technique of LuTaO4 single crystal or ceramic scintillator, which may be applied in the fields of nuclear medicine and high-energy physics. [ABSTRACT FROM AUTHOR]

A complete thermodynamic description of the Na2O-FeO-Fe2O3-Al2O3 system was conducted based on the critical evaluation and optimization of available thermodynamic properties and phase diagram data. The thermodynamic behavior of the liquid solution was described using the Modified Quasichemical Model, and the solid solutions Na(Al,Fe)O2 meta-oxides and Na2(Al,Fe)12O19 Na-β″-alumina were modeled using the Compound Energy Formalism considering their sublattice structures. Most experimental phase diagram data at reducing and oxidizing conditions were reproduced within 3 mol% of compositional error ranges. Many unexplored phase diagrams of this system under different oxygen partial pressures were predicted within the estimated accuracy of ±5 mol% in composition and ±50 K in temperature from the present thermodynamic models with optimized parameters. [ABSTRACT FROM AUTHOR]

As part of the complete thermodynamic modeling of the Na2O-FeO-Fe2O3-Al2O3 system, the Na2O-FeO-Fe2O3-Al2O3 phase diagrams in air (1583 and 1698 K) and at Fe saturation (1573 and 1673 K) were investigated using the quenching method followed by Electron Probe Micro-Analyzer ( EPMA) and X-ray Diffraction ( XRD) phase analysis. General features of the phase diagrams in this system were well revealed for the first time. A complete meta-oxide solid solution between NaAlO2 and NaFeO2 was observed. An extensive solid solution of Na2(Al,Fe)12O19 Na-βˮ-alumina was found and the existence of a miscibility gap in this solution was confirmed. Several compatibility triangles of three-phase assemblages were also identified in air and at Fe saturation. [ABSTRACT FROM AUTHOR]

Magnesium aluminate spinel (MgAl2O4) forms an interesting system having tetrahedral and octahedral voids filled with near similar sized divalent Mg2+ and trivalent Al3+ cations. Structural disorder (e.g., Mg-Al antisite defects) can be tuned by synthetic conditions. This study reports the evolution of Mg/Al disorder in MgAl2O4 prepared by combustion synthesis using different types of fuels. The effect of nature of fuel and the final calcination temperature (600°C-900°C for 9 h) on degree of cation ordering has been investigated combining powder X-ray ( XRD) and neutron ( NPD) diffraction. The results indicate very high degree of inversion in the samples crystallized at low annealing temperature, which on further annealing reduces toward the thermodynamically stable values. Raman spectroscopy, probing MgO4, and AlO4 tetrahedral bonds, confirmed the results at a local level. [ABSTRACT FROM AUTHOR]

Highly crystalline hexagonal titanium disulfide (TiS2) sheets were synthesized at the gas/liquid interface between TiCl4 vapor and the dissolved Na2S in equimolar NaCl- KCl melts at temperatures between 700°C and 850°C. TiS2 sheets with sizes of about 10-20 μm and thicknesses of about 50-200 nm have been collected at 700°C-750°C, and much bigger (50-100 μm in sizes) and thicker (1-5 μm in thickness) TiS2 sheets were obtained at 850°C. The mass transfer preferential growth of an interphase layer and the thermodynamically preferential enlargement of the (001) plane of the hexagonal TiS2 crystal were thought to have led the formation of large and thin TiS2 sheets in this proposed gas/liquid interface process. Due to the density difference, the ripe TiS2 sheets sank into the melt allowing the continuing preparation of TiS2 sheets. These TiS2 sheets could spontaneously assemble to TiS2 membranes or be ultrasonically exfoliated to few-layer TiS2 sheets with thicknesses less than 5 nm. The presented approach should be potentially extendable to the synthesis of large and thin sheets of other layered metal dichalcogenides in large scale. [ABSTRACT FROM AUTHOR]

Mutual solubilities in crystalline phases in the system SiO2-- GeO2 have been investigated up to 10 GPa pressure and 1500°C temperature, using a bulk composition of 50 mol% GeO2. Solid solution of up to 40 mol% GeO2 into the mineral quartz has been confirmed as well as solubility of Si into GeO2rutile (argutite) and Ge into SiO2 rutile (stishovite) phases and limited Ge into coesite. Solubility of Ge in quartz is very high, and decreases with pressure, with the univariant quartz-out reaction occurring near 3.4 GPa at 1200°C. The solubility of GeO2 in coesite is highest at 3.4 GPa (about 8 mol%) and decreases with increasing pressure. Significantly more extensive solubility than previously reported for the rutile phases has been found and measured in detail as a function of pressure and temperature. Extensive solubility of SiO2 in GeO2 is found in argutite at 1200°C, increasing strongly with pressure and reaching a maximum of 25.2 mol% SiO2 in GeO2 at 9 GPa. At this point coesite (ss) plus argutite (ss) react to form a stishovite phase with 18 mol% GeO2, and the mutual solubility in both phases decreases above this pressure. At 1500°C, similar solubilities are observed but the maximum SiO2 solubility in argutite of just over 25 mol% occurs near 10 GPa. All these solid solutions can be recovered to ambient temperature and pressure. Phase diagrams and unit cell information of the phases are presented here. Based on these results, a useful and industrially relevant, application for accurately measuring high pressure is suggested. [ABSTRACT FROM AUTHOR]