The importance of the oxygen ion concentration for oxidic high-temperature solvents

Using e.m.f. measurements we determined relative oxygen ion concentrations of the systems PbO—PbF₂—B₂O₃, PbO—MoO₃—B₂O₃, PbO—Bi₂O₃—V₂O₅, 3 PbO · MoO₃—Fe₂O₃, and 3 PbO · MoO₃—Ga₂O₃. Furthermore the alteration of the relative oxygen ion concentration of pure PbO was observed after stepwise addition of the solvent components PbF₂, B₂O₃ and of the garnet forming oxides Fe₂O₃ and Y₂O₃ giving complete high temperature solutions for the Y₃Fe₅O₁₂ preparation. Applying the Lux' acid-base concept the behaviour of these oxide mixtures is discussed. Combinations which contain weak bases and weak acids allow the adjustment of a desired oxygen ion concentration that is little changed by added oxides for crystal growth. These combinations show favourable properties as solvents for oxidic substances with complicated compositions, e.g. perovskites, garnets, ferrites.     

On the Solubility of Garnets in PbO/B2O3 High Temperature Solvents

The paper is concerned with solubility models for Y₃Fe₅O₁₂ (YIG) and Gd₃Ga₅O₁₂ (GGG) solved in PbO/B₂O₃ high temperature solutions containing Fe₂O₃ or Ga₂O₃ in excess. It is shown that a new extended eight-particle model Interprets well the literature data of the solubility of YIG as well as the DTA data of the solubility of GGG. Comparisons of solution enthalpies determined in terms of the single-molecule, eight-particle, and extended eight-particle model are carried out in dependence on the excess concentration of species containing iron or gallium, respectively.     

Measurement of the shear viscosity in garnet high temperature solutions by a cylindrical rotation viscometer

Garnet high temperature solutions, their solvents and binary systems consisting of lead oxide and one component out of the solute were investigated. The shear viscosity coefficients have been measured in a cylindrical rotation viscometer within the temperature range of liquidus or melting and not exceeding 1270°C. The logarithm of the shear viscosity in the simple systems PbO or PbO B₂O₃ linearily depends on the reciprocal absolute temperature, however, the relationship in phase systems containing Fe₂O₃ or Ga₂O₃ is more complex. The shear viscosities of Y₃Fe₅O₁₂ or (Y, Sm)₃ (Fe, Ga)₅O₁₂ solutions suitable for LPE experiments amount 0.20 to 0.23 g cm⁻¹s⁻¹ near the liquidus temperatures. An effective average activation energy for shearing flow was estimated, its dependence on the temperature points to chemical equilibria reactions in the solutions.     

On the Formation of the Garnet Phase in Oxidic High-Temperature Solutions

The paper informs on the formation of the garnets Y₃Fe₅O₁₂, Y₃Al₅O₁₂ and Gd₃Ga₅O₁₂ as primary phase in the ternary solvent mixtures PbO PbF₂ B₂O₃ and PbO MoO₃ B₂O₃. There are correlations with formerly measured oxygen ion concentrations (OIC) in the solvents:

• i each garnet grows in a special region of the OIC, the phase boundaries are parallel to the lines of constant OIC and
• ii the garnet solubility increases distinctly with decreasing OIC. This emphasizes the important role of the oxygen ions for the garnet formation.

Solubility models are discussed taking into consideration both previous and present results. We conclude that the garnet formation is described best by a model reaction of the type RE rare earth elements, Y; M Al, Fe, Ga.     

Untersuchungen zum Kristallwachstum in der Hochtemperaturlösung mit Hilfe der differentiellen Thermoanalyse

A DTA-apparatus is described which is suitable for solubility measurements of garnets in flux melts. It is important that the temperature will be measured as near as possible to the crystallization region. Solubility temperatures are measured with an uncertainty ≦ 10 deg. A mixture of PbO, PbF₂ and B₂O₃ was used as solvent. The following garnet systems were investigated: Y₃Fe_5−xM_xO₁₂ (M = Ga, Al, In, Cr; x = 0 …︁ 5) and Gd₃Ga₅O₁₂. If results of other authors are available they are in good agreement with our data. Habit and chemical analysis of crystallites give informations about the grown phases and their composition. In connexion with the solubility thus it is possible to select appropriate solvents.     

Luminescence of Y2O3:Eu and Gd2O3:Eu Depending on Precursor and Activation Temperature

The activation of Y₂O₃, Gd₂O₃and (Y_0.7,Gd_0.3)₂O₃ with Eu³⁺ ions at temperatures lower than 1000 °C is studied using different starting compounds. The activator ions are introduced during the crystallization or precipitation of the precursor. Phosphors prepared from hydroxides and activated at 900 °C exhibit luminescence with high efficiency under 254 nm Hg-line excitation. Strong emission is observed even in samples activated at 700 °C. Luminescence intensity, emission and excitation spectra are compared to these of Y₂O₃:Eu produced by the industry.     

Nd3+:Y3Al5O12 laser ceramics: Influence of the size of yttrium oxide particles on sintering

The influence of the size of Y₂O₃ powder particles on the structure formation and densification of Nd³⁺:Y₃Al₅O₁₂ laser ceramics has been studied. It is shown that the use of 50- and 100-nm yttrium oxide particles makes it possible to synthesize single-phase yttrium aluminum garnet at temperatures of 1200 and 1500°C, respectively, whereas in the case of 5000-nm yttrium oxide particles 2-h exposure at a temperature of 1500°C yields only 80 wt % of the Nd³⁺:Y₃Al₅O₁₂ phase. Bulk swelling of pressed samples during sintering of 2.94Y₂O₃-0.06Nd₂O₃-5Al₂O₃ powders with the size ratio of the initial particles R(Al₂O₃/Y₂O₃) ～ 5 is observed. The application of different-sized powders (R ～ 2.5) provides quantitative ratios between phases in the 3Y₂O₃-5Al₂O₃ system at which shrinkage in a temperature range of 20–1500°C is dominant. Laser ceramics 0–2 at % Nd³⁺:Y₃Al₅O₁₂ have been obtained by the solid-phase sintering of oxide powders (R ～ 2.5). The slope efficiency for 1 at % Nd³⁺:Y₃Al₅O₁₂ laser ceramics is found to be 33%.

     

Formation of new lanthanide oxide glasses by laser spin melting and free-fall cooling

Glass spherules were made from molten droplets spun off the ends of laser-heated ceramic rods and solidified by free-falling cooling. Glasses of several hundred μm diameter were formed from the pure lanthanide oxides Sc₂O₃, Y₂O₃, Sm₂O₃, Gd₂O₃, Yb₂O₃ and Lu₂O₃; glass formation decreased with addition of CaO. Stable glasses were also produced from binary La₂O₃ compositions with 25, 50 and 75 wt % of each of the oxides Y₂O₃, Yb₂O₃, Nb₂O₅ and Ta₂O₅, and from the ternary compositions 40 wt % La₂O₃ + 20 wt % CaO + 40 wt % Al₂O₃ or Ga₂O₃. The indices of refraction of all of the glasses and the Abbe numbers of several were measured.     

Spectroscopy of Rare-Earth Elements in Single-Crystal Films of Yttrium-Aluminium Garnets (YAG)

The utilization of liquid-phase epitaxy for growing thin yttrium-aluminium garnet films is discussed. By transfer method from lead solvents films were obtained with a composition Y₃Al₅O₁₂: Nd³⁺ on substrates with composition Y₃Al₅O₁₂, and films Y₃Al₅O₁₂: Er³⁺, Y₃Al₅O₁₂: Er³⁺, Ga³⁺ (about 40 wt% of erbium) on substrates Y₃Al₅O₁₂: Nd³⁺ Characteristics are given for simultaneous induced emission generation of the system film-substrate at 77 K Er³⁺ ion (λ_gen = 1.6602 μ) being a component of the film, and Nd³⁺ ion, being a component of the substrate (λ_gen = 1.061 μm). The results obtained are discussed.     

Thermally activated stress relief in garnet layers grown by liquid phase epitaxy

The lattice misfit of (Lu, Tb)₃Fe₅O₁₂ epitaxial layers grown in compression on Gd₃Ga₅O₁₂ substrates has been found to decrease as a result of annealing at temperatures between 1000 and 1300°C. The deformation rate is thermally activated and depends on the degree of compressive misfit stress at the annealing temperature, the layer thickness and the reducing nature of the anneal atmosphere. Layers which were in tension at the annealing temperature (obtained by using Y₃Fe₅O₁₂ as the substrate material) did not exhibit stress relief. The process has been found to occur nonhomogeneously by the formation of regions of almost total relief which grow and multiply with continued annealing. The shape symmetry of these regions is consistent with dislocation climb in {112} planes. The results are interpreted in terms of a mechanism involving dislocation climb loops (∽1μm diameter) which develop as a result of the formation of oxygen vacancies.     

Band offsets of high dielectric constant gate oxides on silicon

High dielectric constant oxides will soon be needed to replace silicon dioxide as the gate dielectric material in complementary metal oxide semiconductor technology. The oxides must have band offsets with silicon of over 1 eV for both electrons and holes in order to have low leakage currents. We have calculated the band offsets for many candidate oxides using the method of charge neutrality levels. Ta₂O₅ and SrTiO₃ have small or vanishing conduction band offsets on Si. La₂O₃, Y₂O₃, Gd₂O₃, ZrO₂, HfO₂, Al₂O₃ and silicates like ZrSiO₄ have offsets over 1.4 eV for both electrons and holes, making them better gate dielectrics. Zirconates are better than titanates as they have wider gaps.     

X-ray quantitative determination of the phases in a batch for the synthesis of poly- and monocrystal yttriumaluminium garnet

For the synthesis of ceramics and single crystals of yttriumaluminium garnet, a batch consisting of Y₂O₃ and Al₂O₃ is first treated thermally in order to obtain the garnet phase Y₃Al₅O₁₂. It is possible to make a choice of the optimum technological parameters of the batch if the amounts of all oxide phases, present during the synthesis, are well known. A combined X-ray quantitative method is suggested for this purpose. The initial oxides and a garnet phase are determined by the method with external standard. The rest two impurity oxide phases are determined as binary system. The specific diffraction characteristics of the present compounds were taken into account. The amounts of all phases in the batch, treated thermally from 1373 to 2075 K at every 100 K are shown.     

Investigation of Residual Oxide Layers on GaAs Surfaces

GaAs(100) and (111) oriented samples, treated by means of different chemical procedures, are studied by X-ray photoelectron spectroscopy (XPS). The thickness and composition of the residual oxide layers are estimated using the intensity ration of Ga 3d and As 3d core levels from the substrate and the oxide overlayers, respectively. Residual oxides containing As₂ O₃ and Ga₂ O₃ on GaAs surface are observed. The thickness of the oxide layers is found to range from 3Å to 15 Å. Optimal conditions for pre-epitaxial surface preparation of GaAs substrates are suggested.     

Growth and scintillation properties of Pr doped Gd3(Ga,Al)5O12 single crystals

Pr:Gd₃(Ga,Al)₅O₁₂ single crystals were grown by the micro-pulling down (μ-PD) method. All grown crystals were greenish and transparent with 3.0 mm in diameter, 15–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al³⁺ sites by Ga³⁺ in garnet structure has been studied. In these crystals, Pr³⁺ 5d–4f emission is observed with 340 nm wavelength. Pr1%:Gd₃Ga₃Al₂O₁₂ shows highest emission intensity. The light yield of Pr:Gd₃Ga₃Al₂O₁₂ sample with 3 mmφ×1 mm size was around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).     

Study of the influence of additives on some parameters of batches for the synthesis of a light permeable polycrystal of yttrium oxide

For obtaining light permeable ceramic of yttrium oxide, the initial batch should be highly sinterable. The influence of additives of ZrO₂, ThO₂, BeO, Gd₂O₃, Ga₂O₃, La₂O₃ and ThO₂ + Nd₂O₃ on the particle-size of the batch and the occurrence of defects in the unit cell of Y₂O₃ is studied. It has been found that the additives influence the particle-size of the batch and increase the specific surface 4 times. The dependence of the unit cell parameter of Y₂O₃ on the ion radius of the additive is determined by means of X-ray analysis. The measured density of sintered samples of the batches obtained is 99.9–99.96% of the X-ray one and the light permeability at λ = 0.3–10 μm is from 60 to 92%.     

Transport Coefficients and Structural Principles of PbO?Ga2O3 Melt

More Precise knowledge of the growth mechanism of garnets in form of epitaxial layers or single crystals in high temperature solutions on the base of trivalent oxides Me₂O₃ in PbO B₂O₃ solvent is desirable. Data of physicochemical properties of the melts especially density, shear viscosity, and electrical conductivity and their dependence on the temperature are useful in endeavour to recognize the structural principles of oxidie melts. Therefore as a first step the binary system PbO Ga₂O₃ was investigated to understand the influence of the Ga component on a complete mixed garnet solution, for instance (Y, Sm)₃ (Fe, Ga)₅O₁₂ in PbO B₂O₃ solvent.     

Single‐crystal growth and characterization of mullite‐type orthorhombic Bi2M4O9 (M = Al3+, Ga3+, Fe3+)

Pure and homogeneous single crystals of orthorhombic mullite‐type Bi₂M₄O₉ (M = Al³⁺, Ga³⁺, Fe³⁺), and a mixed Bi₂Fe_1.7Ga_2.3O₉ crystal from an equimolar Ga/Fe composition were grown by the top seeded solution growth (TSSG) method. All these compounds melt incongruently in the range of about 800 and 1100 °C. In case of bismuth gallate and ferrate inclusion‐free crystals with dimensions up to several cubic centimeters can be grown. Limited solubility in Bi₂O₃ and the high steepness of the liquidus curve are the reasons for getting only small imperfect bismuth aluminate crystals. In contrast to ceramic materials preparation reported in literature, divalent calcium and strontium could not be incorporated into the mullite‐type structure during the melt growth process. Several fundamental physical properties like heat capacity, thermal expansion, heat conductivity, elastic constants, high‐pressure behavior and oxygen diffusivity were determined by different research groups using single‐crystalline samples from the as‐grown materials. Furthermore, the refractive indices of Bi₂Ga₄O₉ were measured in the range of 0.430 and 0.700 μm. Such as many other bismuth containing compounds the refractive indices of Bi₂Ga₄O₉ are larger than 2, and Bi₂Ga₄O₉ is an optic biaxial positive crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of the temperature and the oxygen partial pressure on the phase formation in the system Cu – Fe – O

Copper iron oxides, Cu_1‐xFe_2+xO₄ (0 ≤ x ≤ 0.5), have been synthesized by thermal oxidation of copper ‐ iron mixtures. In this process, the phase formation and the phase stability were investigated as function of the temperature (800°C – 1200°C) and the oxygen partial pressure (1.013 x 10¹ – 1.013 x 10⁵ Pa). The phase formation starts with the reaction of the metallic components to simple oxides (Fe₃O₄, Fe₂O₃, CuO). From these simple oxides, the formation of complex oxides requires a minimum temperature of 800°C. The synthesis of single phase spinel compounds Cu²⁺_1‐2x Cu¹⁺_xFe_2+xO_4±δ is realized for 0.1 ≤ x ≤ 0.5, using specific temperature – p(O₂) – conditions for a given value of x. Remarkably, to achieve our goal, we found that the increase of x implies that of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. Besides, a single phase spinel CuFe₂O₄ does not exist in the temperature / p(O₂)‐field investigated. Using the results of XRD ‐ phase analysis, T ‐ p(O₂) – x – diagrams were constructed. These diagrams allow the prediction of phase compositions expected for different synthesis conditions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mössbauer study in the glass system PbO · 2B2O3 · Fe2O3

The Mössbauer technique has been employed to study the structure and crystallite formation in the glass system PbO · 2B₂O₃ containing upto 30 wt% Fe₂O₃. Like alkali borate glasses, this glass system also exhibits a broadened quadrupole doublet and iron ions are present in Fe³⁺ state. Above about 20 wt%, the crystallites of magnetically ordered states have been identified. Susceptibility variation with concentration suggests the formation of a superparamagnetic state.     

The effect of B2O3 on the solubility of (YSmLuCa)3(FeGe)5O12 garnet in PbO-B2O3 flux

The paper deals with the solubility of (YSmLuCa)₃ (FeGe)₅O₁₂ garnet in PbO—B₂O₃ flux. An anionic model explaining the effect of B₂O₃ on the solubility of rare earth garnet is suggested. This model is based on the assumption that PbO in the melted mixture of PbO, B₂O₃, Fe₂O₃, R₂O₃, CaO, GeO₂ dissociates into Pb²⁺ and O²⁻. The oxygen anions react with the other oxides under the formation of BO₃³⁻, FeO₃³⁻, RO₆⁹⁻, CaO₂²⁻ and GeO₃²⁻ anions, until the equilibrium is established. The garnet is dissociated into the anions mentioned above. The validity of the model is verified by an agreement of the experimentally determined saturation temperatures of the melts for LPE of garnet layers with the Arrhenius-type expression of the solubility product of garnet where x_i denotes the equilibrium concentrations of the anions in the melt and x° is the Ge content of the garnet film.