Fluorite Solid Solutions Research Articles

Red Pigments Based on CeO2–MO2–Pr6O11(M=Zr and Sn): Solid Solutions for the Coloration of Plastics

New inorganic pigments based on fluorite solid solutions, Ce0.95MxPr0.05−xO2(M=Zr and Sn;xranges from 0 to 0.05) have been prepared by the conventional solid‐state route with a goal of preparing environmentally benign colorants. The pigments obtained displayed better color characteristics than that of Ce0.95Pr0.05O2system showing colors from brick red to dark brown. The coloring mechanism is based on the introduction of an additional electronic energy level of unpaired 4f electron of the praseodymium ion into the cerianite band. Further, the cosubstitution of Zr or Sn in Ce0.95Pr0.05O2system raises the slope of the absorption edge with a blue shift resulting in enhanced lightness. Their applicability for wide spread use, chemical and thermal stability, and capacity to produce pigmentation in plastics, has been investigated. These findings suggest that the developed red pigments may form a viable and potential alternative to traditional toxic red pigments such as cadmium sulfoselenide.

Morphological stability of solid-liquid interface during melt crystallization of M 1−x R x F2+x solid solutions

The physicochemical fundamentals of solid solutions M1 − xRxF2 + x (where M = Ca, Sr, Ba, Cd, and R is rare-earth element) with optical quality without a growth-induced cell substructure are considered. The stability of the plane crystallization front to concentration undercooling without account of released crystallization heat is considered. A modified method of stability functions calculation of the crystallization front using phase diagrams is proposed. For MF-RF3 systems, a critical analysis of experimental data on phase equilibria in the region of existence of 55 fluorite solid solutions M1 − xRxF2 + x is performed and the stability functions are calculated. Coefficients of cation interdiffusion in the melt are estimated by growing concentration series of single crystals.

Lanthanum pyrochlores and the effect of yttrium addition in the systems La 2−xY xZr 2O 7 and La 2−xY xHf 2O 7

The crystal structures of the compounds La 2− x Y x Zr 2O 7 and La 2− x Y x Hf 2O 7 with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La 0.8Y 1.2Zr 2O 7 and La 0.4Y 1.6Hf 2O 7, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48 f ( x, 3 8 , 3 8 ) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.

NaF-CaF2-YbF3 phase diagram

Vertical sections CaF2-Na0.4Yb0.6F2.2 and NaF-Ca0.75Yb0.25F2.25 of the system NaF-CaF2-YbF3 were studied by differential thermal analysis. A fluorite solid solution belt between phases of variable composition Ca1−xYbxF2+x and Na0.5−xYb0.5+xF2+2x was discovered. A liquidus-surface projection was constructed. The solidification character of the solid solution is unfavorable for growth of optical-quality ternary mixed crystals.

Sintering behavior and microwave dielectric properties of Bi 3(Nb 1− xTa x)O 7 solid solutions

Solid solutions of Bi 3(Nb 1− x Ta x )O 7 ( x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h) −1 between 0 ≤ x ≤ 1 and Bi 3(Nb 1− x Ta x )O 7 ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi 3(Nb 1− x Ta x )O 7 ceramics increased from 8.2 to 9.1 g cm −3, microwave permittivity decreased from 95 to 65 while Q f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ f from −113 ppm °C −1 of Bi 3NbO 7 to −70 ppm °C −1 of Bi 3TaO 7. Microwave permittivity, Q f values and τ f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi 3(Nb 1− x Ta x )O 7 ceramics could be a good candidate for low temperature co-fired ceramics application.

Effects of firing schedule on solubility limits and transport properties of ZrO 2–TiO 2–Y 2O 3 fluorites

The low Y/high Zr edge of the cubic defect fluorite solid solution in the system ZrO 2–TiO 2–Y 2O 3 in air is reassessed, as it is these compositions which have been suggested to offer the highest levels of mixed conductivity. Vegard's law is obeyed for values of x which lie within the cubic defect fluorite phase in Zr 1− x − y Y y Ti x O 2− δ for values of y=0.2 and 0.25. Measured lattice parameters show good agreement with those calculated from the Kim relation. Deviation from Vegard's law places the limit of the solid solution at x=0.18 and 0.20 for values of y=0.2 and 0.25, respectively, at 1500 °C. Discrepancies in current literature data can be shown to be due to differences in firing schedule such as slight temperature fluctuations and/or different cooling rates. A high level of care of sintering temperature and cooling profile is essential to form the most promising single-phase materials which contain maximum Ti-contents with low Y-contents. Contraction of the phase limit as a result of poor synthesis control leads to erroneously high values of bulk ionic conductivity while values of electronic conductivity are shown to be less affected.

Preparation and 1.3 .MU.m Emission of Pr3+ Doped Transparent Nano-Glass Ceramics Containing SrF2

Transparent glass ceramics containing fluorite nano-crystals were developed from oxyfluoride glasses containing 32 mol% SrF2 doped with Pr3+ and Yb3+ ions. Materials containing xSrF2+(32-x)CaF2 were also prepared and their fluorescence properties were investigated. Fluorite solid solution crystals containing lanthanide ions were precipitated as a single phase in all the glass ceramic compositions by heat-treatment at appropriate temperatures. The glass ceramics showed Pr3+: 1.3 μm emission, the intensity of which became stronger with increasing SrF2 content and heat-treatment temperature, while only the 1 μm emission of Yb3+ was observed in all the as-made glasses. The lattice constant of fluorite crystals increased with increasing Sr content. Among all the samples, the glass ceramic sample with 32 mol% SrF2 showed the strongest 1.3 μm emission.

Phase Transitions in Binary SnO<sub>2</sub>-CeO<sub>2</sub> System Prepared by a Sol-Gel Method

A differential thermal analyzer (DTA) and a X-ray diffractometer (XRD) were used to study the solid transformations in binary SnO2-CeO2 oxide system. For 100wt% Sn and 100wt%Ce, the transformations from amorphous xerogels to rutile and fluorite start from about 310 and 250°C, respectively. The crystallizations to two-phase mixtures of rutile and fluorite take place at higher temperatures. The two-phase mixtures appear in the region whose composition is between about Sn-10wt%Ce and Sn-75wt%Ce. Sintered at 450°C for one hour, rutile phase dissolves about 10wt% fluorite, and fluorite dissolves about 25wt% rutile. Precipitation reactions take place when the sintering temperature is raised. The phenomenon that precipitation reactions are observed for rutile and fluorite solid solutions means that lower temperature sintering produces solid solutions with high supersaturation. In the as-sintered samples, the grain sizes are fine nano-crystals and the complicated microstructures help the other cations to locate at discontinuous regions.

The order–disorder transition in Bi 2O 3–Nb 2O 5 fluorite-like dielectrics

An analysis of the microwave dielectric properties showed that the cubic fluorite solid solutions (1 − x)Bi 2O 3· xNb 2O 5 (0.23 ≤ x ≤ 0.26), the so-called δ-BN SS Type II, exhibit a high permittivity of 100, but also high dielectric losses ( Q × f = 300 GHz) and a high temperature dependence of resonant frequency ( τ f = −200 ppm/K). Investigations using X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the cubic incommensurate phase undergoes an order–disorder phase transition into a commensurate tetragonal phase at temperatures below 900 °C (for x = 0.25). The higher degree of ordering in the tetragonal phase reduces the dielectric losses ( Q × f = 1000 GHz) and changes the sign of the temperature coefficient of resonant frequency ( τ f = 100 × 10 −6/K).

Mechanochemical Synthesis and Electrical Conductivity of Bi1.6M0.4O3 – x(M = Ca, Ca0.5Sr0.5 , In, Y, La) Metastable Fluorite Solid Solutions

Metastable fluorite solid solutions with the nominal composition Bi1.6Mo0.4O3 – x (M = Ca, Ca0.5Sr0.5 , In, Y, La) are prepared by mechanochemical synthesis, and their structure is studied by x-ray diffraction. The oxygen ion conductivity of dense samples prepared by hot pressing is measured in air.

Understanding Materials Compatibility

▪ Abstract The use of chemical potential diagrams to examine interface chemistry is discussed in terms of the chemical reactions among oxides and associated interdiffusion across the interface. The driving force for both processes can be determined from the chemical potential values. The geometrical features of the chemical potential diagrams can be related to the valence stability of binary oxides and the stabilization energy of double oxides from the constituent oxides. The materials compatibility in solid oxide fuel cell materials is discussed with a focus on a lanthanum manganite cathode and a yttria-stabilized zirconia (YSZ) electrolyte. Emphasis is placed on the valence numbers of manganese in the fluorite solid solution and the perovskite oxides, which have been derived by thermodynamic analysis of the magnitude of the stabilization energy/interaction parameters as a function of ionic size for respective valence numbers. The change of manganese valence on La2Zr2O7 formation and Mn dissolution in YSZ are discussed in relation with the oxygen evolution/adsorption process. Oxygen flow associated with electrochemical reactions exhibits markedly different features depending on the direction of the polarization, which can lead to drastic changes in the interface chemistry (precipitation or interdiffusion).

Subsolidus phase equilibria in the RuO2–Bi2O3–CeO2 system

Subsolidus equilibria in air in the RuO2–Bi2O3–CeO2 systems were studied with the aim of obtaining information on possible interactions between a Bi2Ru2O7-based cathode and a CeO2-based solid electrolyte in solid-oxide fuel cells. Bi2O3 is soluble in CeO2, and forms a cubic fluorite solid solution Bi1-xCexO2-x/2 up to Bi1/3Ce2/3O1.83, while no solid solubility of the CeO2 in Bi2O3 was detected. No ternary compound was found in the system. The tie line is between Bi2Ru2O7 and the CeO2 solid solution.

Mechanochemical synthesis and thermal behavior of metastable mixed oxides in the CaO-Sb2O3-Bi2O3 system

The phase composition of crystalline mechanochemical synthesis products in the CaO–Sb2O3–Bi2O3 system was determined. Of the known phases in this system, only three could be prepared mechanochemically: Ca2Sb2O5, CaSb2O4, and ∼CaBiO2.5 (fcc). A new metastable phase, δ"-Bi2O3, with an orthorhombic structure close to that of the high-temperature, fluorite phase δ-Bi2O3, was obtained by mechanical processing at ≃30°C. A number of new metastable fluorite solid solutions of binary and ternary oxides were obtained as single-phase powders by mechanochemical synthesis. The mechanochemical yield of primary crystalline products was shown to be several times higher than that of secondary products. A broad composition range was revealed in which perovskite and fluorite phases are in mechanochemical equilibrium. The composition dependence of the lattice parameter of the metastable fluorite phase Bi2 – x Sb x O3 was found to be the opposite of the one predicted by Vegard's law. Metastable mixed oxides undergo phase transformations during heating (starting at 280°C in the case of the ternary perovskite phase). Bi2 – x Ca x O3 – 0.5x fluorite solid solutions experience a transformation at ≃400°C, accompanied by oxygen loss. During heating in air, Sb2O3-containing fluorite phases partially stabilize owing to oxidation but, nevertheless, undergo structural transformations above 480°C. The transformation of Sb2 – x Ca x O3 – 0.5x metastable fluorite solid solutions near 500°C in air is accompanied by the formation of needle-like Sb2O3 crystals. A mechanism is proposed for the extremely rapid growth of such crystals: extrusion of the Sb2O3 resulting from fluorite decomposition in agglomerates through triple junctions of aggregates and through cracks in the surface layer of agglomerates.

Phase Study and Thermal Expansion of Yttria Stabilized Zirconia Doped with PuO2-x

Phase relations between PuO2 and yttria stabilized zirconia (YSZ, Pu/(Pu+YSZ)=0-0.5) were investigated in air, vacuum and H2 atmospheres. All samples prepared in air and vacuum were single-phase solid solutions with the fluorite structure while those prepared in H2 were single-phase either the fluorite (Pu/(Pu+YSZ)<0.3) or the pyrochlore structure (Pu/(Pu+YSZ)>0.3). The observed lattice parameter change of the fluorite solid solution with the Pu content was well explained by an ionic model where oxygen vacancies occurred preferentially to Zr4+ and the coordination number (CN) of Zr4+ reduced to 7 while the CN of the other metal ions remained 8. Thermal expansion of YSZ doped with PuO2 was also studied by high-temperature X-ray diffraction method at temperatures between room temperature and 1273 K in air. Linear thermal expansion coefficient values of the solid solution (Pu/(Pu+YSZ) = 0-0.25) fell in the range 6-12 x 10-6 K-1. Though there was no apparent relationship between thermal expansion coefficients and Pu content, it was seen that thermal expansion coefficients became small with increasing Pu content above 1000 K. The constant-pressure heat capacities of the solid solutions were also calculated from the thermodynamical relation.

Anion Transport in Superionic Conductors Na0.5 – xR0.5 + xF2 + 2x (R Are Cations of Rare-Earth Elements) at Elevated Temperatures

Anion conduction in solid solutions Na0.5 – xR0.5 + xF2 + 2x (R = Dy; Ho; Tm; Y; combinations Y0.9Nd0.1, Y0.5Yb0.5, Lu0.998Ho0.002, Lu0.9Ho0.1, Lu0.9Nd0.1) with a fluorite structure is studied at 251–1073 K. The ion transport mechanism alters at Tc ≈ 723–773 K. Below Tc the mechanism involves migration of interstitial fluorine atoms and depends mainly on the defect-containing (clustered) structure of the solid solutions. Above Tc the mechanism probably involves the dissociation of interstitial fluorine atoms from structural clusters, and the anion conductivity reaches ∼1 S cm–1. The fluorite solid solution in the Na0.4Ho0.6F2.2 crystals partly decomposes, yielding NaHoF4 compounds with a gagarinite (NaCaYF6) structure.

Structural studies on the optimisation of fast oxide ion transport

Extensive regions of cubic fluorite solid solution formation are observed in the Y 2O 3–ZrO 2, Y 2O 3–ZrO 2–TiO 2 and Y 2O 3–ZrO 2–Nb 2O 5 systems. Ionic conductivity studies indicate that the activation energy for conduction increases rapidly whilst the high temperature conductivity decreases as yttria-content increases. At high defect concentrations, however, activation energies tend towards a constant value. The increase in activation energy correlates with an increase in the extent of diffuse reflections in neutron diffraction experiments and with increases in the isotropic temperature factor associated with the anion sublattice. Significant negative correlation was observed between conductivity prefactor and oxygen temperature factors. Electron diffraction experiments also show diffuse elements in addition to the cubic fluorite structure, indicating the presence of short-range order related to C-type and distorted pyrochlore types.

Subsolidus phase equilibria in the NiO–CeO 2 and La 2O 3–CeO 2–Fe 2O 3 systems

Subsolidus equilibria in air in the NiO–CeO 2 and La 2O 3–CeO 2–Fe 2O 3 systems were studied. In the NiO–CeO 2 and Fe 2O 3–CeO 2 systems, no binary compound and no solid solubility were detected. La 2O 3 was soluble in CeO 2, forming a cubic fluorite solid solution up to La 0.5Ce 0.5O 1.75, whereas no solid solubility of CeO 2 in La 2O 3 was detected. In the La 2O 3–CeO 2–Fe 2O 3 system, no ternary compound was found. The tie line is between LaFeO 3 and CeO 2.

Formation of Metastable Fluorite Phase by Rapid Quenching of Melts in the Y2O3‐Ta2O5 System

Melts of x mol% Ta2O5–Y2O3 (x = 0–32.5) were rapidly quenched to investigate the formation of metastable fluorite solid solutions. C‐type Y2O3, fluorite, and fergusonite phases existed in the compositional regions of 0 x 16, 8 x 32.5, and 27.5 x 32.5, respectively. Their lattice parameters were precisely measured through either Rietveld analysis or a least‐squares fit of the individual X‐ray diffraction peak positions. The lattice parameter of the fluorite phase decreased linearly with increasing Ta2O5 content, strongly suggesting the formation of compositionally homogeneous metastable solid solutions. Ta2O5 was almost insoluble into Y2O3 at 1700°C in the equilibrium state.

High‐Temperature Electrical Properties in the ThO2‐Rich Region of ThO2‐RE2O3 Systems

In the present study the n‐type electronic conduction in terms of the parameter pe, and the phase relations in several ThO2‐RE2O3 systems were examined. Large fluorite solid solution regions exist at elevated temperatures. It was demonstrated that RE2O3‐doped thoria compositions feature lower parameters pe, and higher chemical stability than the conventional stabilized ZrO2 electrolytes. The results are given in terms of the characteristic parameter pe, in the temperature range from 1000° to 1600°C. The experimental investigations were made using a new thermodynamic measuring system.

Phase Equilibria and Ordering in the System Zirconia‐Hafnia‐Yttria

The phase equilibria in the system zirconia‐hafnia‐yttria were investigated at 1300° and 1600°C by precise lattice parameter measurements and the direct phase‐inspection method. Large fluorite solid solution regions were dominant at both investigated temperatures. The extent of ordering within the ternary system was studied in the samples containing 40 mol% yttria at 1300°C. The δ‐phase (Zr,Hf)3Y4O12 formation was established only in those samples with less than 15 mol% hafnia.