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The kinetics of formation and crystallization of Ln2Ti2O7 (Ln = Gd, Lu) pyrochlores from nanoparticulate precursors prepared via coprecipitation have been analyzed using X-ray diffraction, Raman spectroscopy, luminescence spectroscopy, scanning electron microscopy, calorimetry (differential scanning calorimetry (DSC)), mass spectrometry, and quantitative thermogravimetry (TG). The results demonstrate that the formation of the pyrochlores proceeds through crystallization of a nanoparticulate fluorite phase. The starting mixtures have been shown to consist in considerable measure of hydroxides and hydroxycarbonates, rather than of oxides. The first synthesis step at temperatures below 550–650°C is decomposition of the starting compounds to titanium oxide and lutetium (or gadolinium) dioxycarbonate. The second step is the synthesis of final compounds, also accompanied by CO2 release. Thus, "high-temperature" CO2 release makes it possible to visualize the synthesis kinetics. Specially designed experiments involving prolonged heat treatment at low temperatures (540 h at 550°C and 216 h at 700°C) have shown that Lu2Ti2O7 can be synthesized almost completely from starting mixtures even at 550°C. The high degree of conversion at 550°C has been confirmed by quantitative TG. Raman and luminescence spectroscopy results demonstrate that the disordered nanooxide synthesized at 550°C, with a crystallite size of 15 Å, has the fluorite structure. Prolonged heat treatment at 700°C was accompanied by an increase in crystallite size and a fluorite-to-pyrochlore phase transition. During heating of the starting precursor at a high rate, 10°C/min (DSC and TG), all of the processes were shifted to higher temperatures. The formation of the final Ln2Ti2O7 (Ln = Gd, Lu) pyrochlores through a nanoparticulate fluorite phase is characteristic of both systems. Thus, all of the Ln2M2O7 (M = Ti, Zr, Hf) 3+/4+ pyrochlores are formed as a result of an order–disorder transition from a nanoparticulate fluorite phase to a pyrochlore phase. [ABSTRACT FROM AUTHOR]

Praseodymium hafnate (Pr2Hf2O7) powder with the pyrochlore structure has been synthesized and characterized by X-ray diffraction, scanning electron microscopy, and chemical analysis. Its molar heat capacity has been determined in the temperature range 315–1345 K and its lattice parameter has been measured as a function of temperature in the range 298–1273 K. [ABSTRACT FROM AUTHOR]

Dense Gd2Zr2O7-based ceramics with the composition Gd2Zr1.9Be0.1O6.9, containing heterovalently substituted beryllium on the zirconium site, have been prepared for the first time via mechanical activation of an oxide mixture, followed by annealing of green compacts at 1500°C. The use of mechanical activation has been shown to enable the preparation of dense ceramics (relative density of 88%) by annealing for 5 min and gas-tight ceramics (relative density of 97.3%) by annealing for 4 h. Gd2Zr1.9Be0.1O6.9 has a pure oxygen ion conductivity of 4.0 × 10–3 S/cm at 800°C. The main advantage of partial beryllium substitution on the zirconium site in Gd2Zr2O7 is the formation of a material having negligible electron and hole conductivities in wide ranges of oxygen partial pressures and temperatures. [ABSTRACT FROM AUTHOR]

Raman spectra of zirconia doped with 10, 20, and 25% ytterbia have been measured using light sources with wavelengths of 785 and 532 nm. A number of bands have been shown to depend on the laser wavelength used, that is, they cannot be Stokes bands. Analysis of Stokes bands has made it possible to describe the structure of the samples containing 10 and 20% ytterbium as a defect pyrochlore structure, which points to changes in the local symmetry of oxygen around cations on cooling to room temperature. [ABSTRACT FROM AUTHOR]

Dense Gd2Zr2O7-based ceramics with the composition Gd2Zr1.9Be0.1O6.9, containing heterovalently substituted beryllium on the zirconium site, have been prepared for the first time via mechanical activation of an oxide mixture, followed by annealing of green compacts at 1500°C. The use of mechanical activation has been shown to enable the preparation of dense ceramics (relative density of 88%) by annealing for 5 min and gas-tight ceramics (relative density of 97.3%) by annealing for 4 h. Gd2Zr1.9Be0.1O6.9 has a pure oxygen ion conductivity of 4.0 × 10–3 S/cm at 800°C. The main advantage of partial beryllium substitution on the zirconium site in Gd2Zr2O7 is the formation of a material having negligible electron and hole conductivities in wide ranges of oxygen partial pressures and temperatures. [ABSTRACT FROM AUTHOR]

The molar heat capacity and lattice parameter of samarium hafnate with the pyrochlore structure have been measured as functions of temperature in the ranges 320–1300 and 298–1273 K, and its thermal expansion coefficients have been evaluated. [ABSTRACT FROM AUTHOR]

This paper reports the synthesis of compounds with the pyrochlore and hexagonal tungsten bronze structures via thermal decomposition of heteropolyoxometalates. Using aqueous solutions, we have synthesized tungstophosphatometalates with the Keggin structure and the general formula Ct5[PW11O39(H2O)Z]⋅nH2O, where Ct = Rb+ or Cs+ and Z = Co2+, Ni2+, or Cu2+. We have studied the thermal decomposition of these compounds and identified their thermolysis products: phases with the pyrochlore and hexagonal tungsten bronze structures. Our results confirm that phosphorus, cobalt, nickel, and copper ions become incorporated into the pyrochlore and hexagonal tungsten bronze structures of the CtnxPxZxW2–2xO6 compounds. No phases with similar chemical compositions have been reported previously. Their synthesis temperature has been lowered by 200°C and the calcination time has been reduced by a factor of 2 in comparison with conventional synthesis methods. The proposed schemes of thermolysis of rubidium and cesium tungstophosphatometalates will be useful for predicting the thermal properties and phase composition of thermolysis products of analogous heteropolyoxometalates in designing new inorganic materials based on them. [ABSTRACT FROM AUTHOR]

We have studied the effect of mechanical activation in lead ferroniobate synthesis on the formation of perovskite and pyrochlore phases during both mechanochemical synthesis and subsequent firing. It has been shown that, during mechanical activation, the first to form is the perovskite structure. During subsequent sintering of the material, the pyrochlore structure begins to form as well. As the firing temperature is raised to 650–750°C, the perovskite structure is again formed. We provide explanation for this process. [ABSTRACT FROM AUTHOR]

Ln2O3 + HfO2 (Ln = Nd, Dy) powders and ceramics have been studied in an oxidizing (O2) and a mild reducing (He) atmosphere using differential scanning calorimetry (DSC), thermogravimetry, mass spectrometric analysis of released gases, X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The results demonstrate that both a mechanically activated oxide mixture of appropriate composition and the powders and ceramics prepared by heat-treating the mixture contain carbon-containing compounds (basic rare-earth carbonates and hydroxycarbonates) and/or at least 0.2–0.5 wt % carbon (X-ray amorphous or crystalline). As a result, during heating in an oxidizing atmosphere all of the samples release CO2 in the same temperature ranges (250–600 and 750–1200°C), which is accompanied by exothermic peaks in their DSC curves. The CO2 release in the range 250–600°C is due to the onset of decomposition of the basic rare-earth carbonates and hydroxycarbonates, which are present in small amounts in the starting mixture, powders, and ceramics. The CO2 release in the range 750–1200°C is due to the burnout of strongly bonded carbon and thermally stable carbon-containing compounds (rare-earth dioxymonocarbonates, Ln2O2CO3). The exothermic peaks in the DSC curve are due to fluorite LnHfO4 – δ (Ln = Nd, Dy) crystallization processes. We believe that synthesis in air, involving the formation of X-ray amorphous (fine-particle and nanocrystalline) precursors containing rare-earth oxides, which tend to form basic rare-earth carbonates and hydroxycarbonates in air, will always yield high-temperature ceramics containing carbon compounds and at least 0.5 wt % X-ray amorphous carbon and/or graphite. The amount of carbon and carbon-containing compounds in the dysprosium-containing ceramics is markedly smaller (~0.2%) than that in the neodymium-containing ceramics. The crystallization of the rare-earth hafnates is a rather slow process that can begin at temperatures as low as 550°C. The formation of Nd2Hf2O7 with the pyrochlore structure involves fluorite NdHfO4 – δ formation as an intermediate step, and a single-phase product can only be obtained by high-temperature firing at ~1600°C. Phase-pure DyHfO4 – δ with the fluorite structure can be obtained by firing at 1200°C. [ABSTRACT FROM AUTHOR]

Ceramic materials based on the Bi2ZnNb2O9 and Bi2(Zn1/3Nb2/3)2O7 phases with the pyrochlore structure in the Bi2O3–ZnO–Nb2O5 system have been prepared by solid-state reactions, and their phase composition, crystal structure, and electrical properties have been studied. We have examined the effect of glass additions on the sintering temperature and electrical properties of the ceramics and obtained thermally stable ceramic materials with ε ~ 100 and low dielectric losses in the frequency range from 0.02 to 2 MHz. They have a sintering temperature below 950°C and are potentially attractive for use in multilayer structures with a silver-based electrode. [ABSTRACT FROM AUTHOR]

(Ln1.8Fe0.2)FeSbO7 (Ln = Pr–Tb) mixed oxides have been synthesized via coprecipitation followed by annealing. According to Rietveld refinement results, all of the oxides have the pyrochlore structure (sp. gr. Fd mz). The (Ln1.8Fe0.2)FeSbO7 compounds have been shown to exhibit high activity for CO oxidation reaction. The best results have been obtained for (Pr1.8Fe0.2)FeSbO7, which has a 90% CO conversion temperature T90 = 385°C. With increasing lanthanide atomic number, T90 rises monotonically from 385 to 550°C in going from Pr to Tb. This behavior is most likely due to the decrease of structural imperfection in the compounds with smaller lanthanide ionic radii. [ABSTRACT FROM AUTHOR]

RSnO (R = Pr-Lu) rare-earth stannates with the pyrochlore structure have been synthesized by solid-state reactions, by firing stoichiometric mixtures of SnO and RO in air at 1473 K. The high-temperature heat capacity of the rare-earth stannates has been determined by differential scanning calorimetry in the temperature range 350 to 1000 K, and the Raman spectra of polycrystalline TbSnO and DySnO samples have been measured. [ABSTRACT FROM AUTHOR]

A technique has been developed for fluorinating the pyrochlore oxide BiFeFeSbO, and a compound with the composition BiFeSbOF has been obtained. The synthesized oxyfluoride also has the pyrochlore structure (sp. gr. Fd3 m), with a lattice parameter a = 10.4443(1) Å ( R = 5.2). It has been shown that the charge balance upon fluorine substitution for oxygen is maintained not through partial reduction of Fe to Fe but through the incorporation of fluorine into oxygen vacancies. The magnetic behavior of the fluorinated pyrochlore phase is determined by the persisting frustration of the octahedral sublattice, which is responsible for the development of a spin glass state below T = 12 K. The fluorination-induced changes in the anion sublattice led to an increase in the antiferromagnetic exchange interaction between neighboring Fe ions and changes in the dynamic properties of the spin glass phase. [ABSTRACT FROM AUTHOR]

YbSnO and LuSnO have been prepared by solid-state reactions, by firing mixtures of YbO or LuO and SnO at 1473 K, and the molar heat capacity of these compounds (pyrochlore structure) has been determined by differential scanning calorimetry. The C ( T) data have been used to evaluate the thermodynamic properties of the stannates: enthalpy increment, entropy change, and reduced Gibbs energy. [ABSTRACT FROM AUTHOR]

The Bi-La-Fe-Sb-O, Pr-Fe-Sb-O, and Bi-Pr-Fe-Sb-O systems have been shown to contain pyrochlore solid solutions. We have determined the limits of the PrFeSbO and BiLnFeSbO (Ln = La, Pr) solid solutions and shown that there is no compound corresponding to the 'ideal' composition PrFeSbO. Analysis of the magnetic properties of the synthesized compounds indicates that rare earth substitution for bismuth ions has no effect on the spin glass transition and that the spin glass state is only determined by the interaction between the Fe spins on the octahedral site. The Pr paramagnetic ion in PrFeSbO does not participate in the formation of the spin glass state but makes a paramagnetic contribution to the total magnetic susceptibility, which is especially significant at low temperatures. [ABSTRACT FROM AUTHOR]

Coatings based on lanthanum, neodymium, samarium, and gadolinium zirconates have been grown by atmospheric plasma spraying of powders, and their composition and structure have been investigated by scanning electron microscopy, chemical analysis, and X-ray diffraction. The results demonstrate that the chemical composition of the coatings differs from the compositions of the powders they were prepared from. During plasma spraying of LnZrO-based powders, the rare-earth oxide vaporizes more rapidly than zirconia. The difference in composition between the powders and coatings decreases as the atomic number of the rare earths increases in going from La to Gd. We have studied the processes that take place in the coatings during heat treatment at a temperature of 1250°C. It has been shown that the major phase in the LaZrO-, NdZrO-, and SmZrO-based coatings undergoes a defect fluorite-pyrochlore structural phase transition. The LaZrO-based coatings have been found to contain ∼5 wt % t-ZrO in addition to the major phase. The GdZrO-based coating retained a defect fluorite structure. [ABSTRACT FROM AUTHOR]

Order-disorder transitions in xRO · (1 − x)TiO (R = Sc, 0.4 ≤ x ≤ 0.5; R = Y, 0.5 ≤ x ≤ 0.6) solid solutions with highly imperfect fluorite-derived structures have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of a fluorite-like ( Fm3 m) disordered phase and a nanoscale (~10-100 nm) pyrochlore-like ( Fd3 m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The Raman spectra of ScTiO (YTiO) contain broad lines in low- and high-frequency regions: at 190, 350, and 775 (134, 188, 365, 404, and 727) cm. These lines are characteristic of a pyrochlore-like phase with a varying degree of order and a disordered fluorite-like phase, respectively. The pyrochlore-like phase YTiO has two strong Raman peaks in the low-frequency region: at 312 and 527 cm. The formation of nanodomains with different degrees of order is caused by the internal stress that arises from the high density of structural defects in the unit cells of the solid solutions. [ABSTRACT FROM AUTHOR]

Using high-pressure heat treatment of amorphous low-hydrated tantalum hydroxide of composition TaO(OH) · 1.0-2.5HO at p = 5.0 GPa and t = 600-750°C, we obtained a mixture of two phases: HTaO · HO with a defect pyrochlore structure (sp. gr. Fd3m, a = 10.5956(1) Å, Z = 8, V = 1189.53(3) Å) and TT-TaO (δ) (α-UO structure, sp. gr. P3 m1, a = 3.6368(2) Å, c = 3.8943(5) Å, Z = 1, V = 44.61 Å). The X-ray diffraction data for these compounds were analyzed by the Rietveld method. The agreement factors obtained are R = 0.0541 for HTaO · HO and R = 0.0885 for TT-TaO. These phases are assumed to form an intergrowth structure along certain crystallographic planes. We have proposed a mechanism of intergrowth structure formation in the [111] direction of the pyrochlore structure and in the [001] direction of TT-TaO. [ABSTRACT FROM AUTHOR]

X-ray diffraction characterization with monochromatic synchrotron X-rays has demonstrated that xScO · (1 − x)TiO ( x = 0.4-0.5) solid solutions consist of a fluorite-like ( Fm3 m) disordered phase and a nanoscale (≃ 10-50 nm) pyrochlore-like ( Fd3 m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The formation of nanodomains with different degrees of order is shown to be caused by the internal strain due to the high density of structural defects in their unit cells. The materials obtained in this study possess enhanced sorption capacity and can be used as catalysts, catalyst supports, gas sensors, etc. [ABSTRACT FROM AUTHOR]

We have compared (LnZr)ZrO (Ln = Nd, Sm) pyrochlore-like solid solutions with interstitial oxide ion conduction and Ln(ZrLn)O (Ln = Nd, Sm) pyrochlore-like solid solutions with vacancy-mediated oxide ion conduction in the symmetric systems NdO-ZrO (NdZrO) and SmO-ZrO (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (SmZr)ZrO ( x = 0.2) material using thermal analysis and mass spectrometry in a reducing atmosphere (H/Ar-He). The LnZrO (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3-4) × 10 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln(ZrLn)O (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion conductivity of the (LnZr)ZrO (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlore-like solid solutions is essentially constant in a broad range of LnO concentrations. [ABSTRACT FROM AUTHOR]

We have studied the accommodation of niobium and cerium variable valence cations in the LnTiO (3+/4+) (Ln = Dy, Yb) pyrochlores in air and in a 5% H + 95% Ar reducing atmosphere. The (DyCaCe)TiO and (YbTbCa)[TiNb]O ( x = 0-0.3) oxygen ion conducting solid solutions were synthesized through coprecipitation followed by heat treatment in air for 4 h at temperatures of 1400 and 1550°C, respectively. According to X-ray photoelectron spectroscopy (XPS) data, the valence state of the niobium in the (YbTbCa)[TiNb]O ( x = 0.05, 0.1) solid solutions is 4+ and, hence, isovalent niobium substitution for titanium takes place. During synthesis in air, the titanium sublattice of the (YbTbCa)[TiNb]O pyrochlores is stable when the Nb content does not exceed 10% ( x = 0.1). According to conductivity data for the high-conductivity material (YbTbCa)[TiNb]O at low oxygen partial pressures, there is no reduction of pentavalent niobium and no increase in electronic conductivity, which is supported by XPS results that only Nb is present in the solid solution. In pyrochlore-like (DyCaCe)TiO synthesized in air at 1400°C, the dysprosium site contains both tri- and tetravalent cerium, as supported by conductivity measurements under reducing conditions. The Ce + e → Ce reduction process led to a change in the color and disintegration of the sample when the oxygen partial pressure was reduced. In the 3+/4+ ABO pyrochlores, variable valence cations on the A site (eightfold coordination) more readily participate in redox processes than do those on the B site (sixfold coordination). [ABSTRACT FROM AUTHOR]

We have studied terbium substitution for ytterbium in (YbTbCa)TiO ( x = 0.1, 0.2, 0.3, 0.4) pyrochlore solid solutions synthesized through coprecipitation followed by firing at 1550°C. The results indicate that only a small amount of terbium (less than 10%) can be incorporated into the pyrochlore structure of (YbCa)TiO because of the large difference in ionic radius between the terbium and ytterbium cations: Δ r = r(Tb) − r(Yb) = 0.055 Å. The oxygen ion conductivity of the (YbTbCa)TiO solid solutions has been determined by impedance spectroscopy in air in the temperature range 300 to 900°C. At high temperatures ( t > 640°C), their bulk conductivity was essentially independent of the Yb/Tb ratio. The observed decrease in density and microstructural changes were insignificant. At relatively low temperatures ( t < 640°C), the bulk conductivity decreased slightly, and the decrease depended little on terbium concentration. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the pyrochlore-to-perovskite phase transition in ferroelectric lead zirconate titanate (PZT) films grown on silicon substrates by rf magnetron sputtering and annealed in air or in an inert (argon) atmosphere at temperatures of up to 600°C and atmospheric pressure. Simultaneous thermal analysis results demonstrate that annealing in air leads to release of the latent heat of the phase transition, which is due to the conversion of lead oxide to lead orthoplumbate in the bulk of the PZT film. This transition is accompanied by changes in the densities of the perovskite phase and parent (pyrochlore) phase. In this case, the possibility of phase transformation should be ensured by changes in the volume of the system. The change in the volume of the film leads to the formation of micropores in the bulk of thin PZT films. The micropore size has been determined by scanning electron microscopy, and the phase composition of the films has been assessed by X-ray diffraction. We present experimental evidence that micropore nucleation and growth at the interface between the new and old phases in thin PZT films is due to the difference in density between these phases. [ABSTRACT FROM AUTHOR]

GdSnO gadolinium stannate with the pyrochlore structure has been prepared by solid-state reaction and its high-temperature heat capacity has been determined by differential scanning calorimetry in the temperature range 350-1020 K. The C( T) data are shown to be well represented by the classic Maier-Kelley equation. The experimental C( T) data have been used to evaluate the thermodynamic functions of gadolinium stannate: enthalpy increment H°( T)- H°(339 K), entropy change S°( T)- S°(339 K), and reduced Gibbs energy Ф°( Т). [ABSTRACT FROM AUTHOR]