Abstract: The Heisenberg nearest neighbour antiferromagnet on the pyrochlore (3D) lattice is highly frustrated and does not order at low temperature where spin-spin correlations remain short ranged. Dzyaloshinsky-Moriya interactions (DMI) may be present in pyrochlore compounds as is shown, and the consequences of such interactions on the magnetic properties are investigated through mean field approximation and monte carlo simulations. It is found that DMI (if present) tremendously change the low temperature behaviour of the system. At a temperature of the order of the DMI a phase transition to a long range ordered state takes place. The ordered magnetic structures are explicited for the different possible DMI which are introduced on the basis of symmetry arguments. The relevance of such a scenario for pyrochlore compounds in which an ordered magnetic structure is observed experimentally is dicussed.

Abstract: Phase equilibrium studies of the Bi–Zn–Nb–O system show that pyrochlore does not form at chemical compositions predicted by the traditional formula for this crystal structure, A2B2O6O', where A denotes large (8-coordinated, e.g. Bi3+) and B small (6-coordinated, e.g. Zn2+, Nb5+) cation sites. Instead, pyrochlore forms only at compositions with excess B cations which, surprisingly, occupy the large A-cation sites. Reports of similar behavior in other pyrochlores suggest a previously unrecognized inherent structural feature (displacive disorder) which allows the formation of a large family of cubic pyrochlores with small B cations occupying up to ca. 25 % of the large A-cation sites. Many pyrochlores can now be synthesized by deliberately combining large and small metal ions on the A sites. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Abstract: Single-phase perovskite 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 (0.9PMN–0.1 PT) from a stoichiometric mixture of starting materials was synthesized by applying a mechanochemical technique to the stage of a precursor. A stoichiometric mixture of PbO, TiO2, Mg(OH)2, and Nb2O5 was milled for 60 min and heated at temperatures as low as 850°C for 4 h to obtain a single phase. The maximum dielectric constant of the samples from the milled mixture increased as the sintering temperature increased, with the remarkable grain growth, and attained 24600 at 1200°C. In contrast, poor densification and coexistence of the pyrochlore phase were observed on the samples from the nonmilled mixture. Further observation suggested that the pyrochlore phase concentrated near the surface during sintering and then migrated into the PbZrO3 packing powder, leading to a pyrochlore–free phase at 1250°C. The dielectric constant of the latter ceramics was explained by the series mixing rule for the dielectric constant of a diphasic solid.

Abstract: The change in permittivity of bismuth zinc niobate (BZN) films with the cubic pyrochlore structure under an applied electric field was measured as a function of temperature. Dielectric measurements were performed using parallel-plate capacitor structures with Pt electrodes on sapphire substrates. The electric field tunability of the permittivity was weakly temperature dependent and increased with decreasing temperature up to the onset of dielectric relaxation. At temperatures below the onset of the dielectric relaxation (similar to150 K at 1 MHz), larger electric fields were required to achieve the highest tunabilities. A simple model of hopping, noninteracting dipoles was not suited to describe. the dielectric tunability of BZN thin films. A better description of the experimentally observed behavior at temperatures above the onset of the dielectric relaxation was obtained using a simple random-field model with hopping dipoles in a uniform distribution of random fields. (C) 2005 American Institute of Physics.

Abstract: We measure pseudodielectric functions in the visible-deep ultraviolet spectral range of Pb(ZrxTi1−x)O3 (x=0.2,0.56,0.82) (PZT), Pb0.98Nb0.04(Zr0.2Ti0.8)0.96O3, Pb0.91La0.09(Zr0.65Ti0.35)0.98O3, and Pb0.85La0.15Ti0.96O3 films grown on platinized silicon substrates using a sol-gel method and on (0001) sapphire using a radio-frequency sputtering method. Using a parametric optical constant model, we estimate the dielectric functions (ϵ) of the perovskite oxide thin films. Taking the second derivative of the fitted layer dielectric functions and using the standard critical-point model, we determine the parameters of the critical points. In the second derivative spectra, the lowest band-gap energy peak near 4eV is fitted as a double peak for annealed PZTs due to the perovskite phase. As-grown PZTs have mainly pyrochlore phase and the lowest band-gap peak is fitted as a single peak. We also examine the effect of dopants La and Nb, which substitute at Pb and Zr (Ti) sites, respectively. We found three band gaps E...

Abstract: Two β-pyrochlore oxide superconductors, CsOs 2 O 6 and RbOs 2 O 6 , are studied thermodynamically by measuring specific heat on polycrystalline samples. It is found that a Sommerfeld coefficient γ is nearly equal, 20 mJ/K 2 mol Os, in the two oxides with different superconducting transition temperatures; T c =3.3 and 6.3 K, respectively. This suggests that the density of states at the Fermi level is not a crucial parameter to determine the T c of the β-pyrochlore oxide superconductors, which is incompatible with the general expectation for a conventional BCS-type superconductor. Anomalous lattice contributions to specific heat at low temperature are also reported, which may come from nearly localized phonon modes associated with the rattling of the alkali metal ions weakly bound in an oversized cage formed by OsO 6 octahedra.

Abstract: Reaction between rare-earth tantalates RTaO4 and ammonia flow at 900−950 °C forms oxynitrides belonging to different structure types, perovskites RTaON2, pyrochlores R2Ta2O5N2, and defect fluorites RTa(O,N,□)4, depending on the size of the R element. The nature of the oxide precursor is a crucial parameter affecting the ammonolysis reaction. A comparative study has been carried out between oxide powders prepared by a ceramic route and a chimie douce process. The pyrochlore structure is restricted to large rare-earths (R = Nd→Gd). For smaller R elements, thermal ammonolysis of reactive precursors elaborated from the citrate combustion route results in a fluorite-type oxynitride solid solution. X-ray and neutron diffraction studies evidence a totally disordered cubic fluorite unit cell, which, however, appears to be not suitable for the structure refinement, thus revealing a more complex atom arrangement.

Abstract: A method for the preparation of Cr-doped Y2Sn2O7 powders based on the pyrolysis of aerosols generated from aqueous salt solutions of the corresponding metal cations is reported These powders yielded Cr-doped pyrochlore red-shade pigments on calcination in the absence of fluxes at lower temperature (900°–1100°C) than that involved in the traditional ceramic method (1400°C) This process resulted in pigment particles with spherical shape and broad size distribution (<10 μm) not requiring milling The oxidation state of the Cr ions dissolved in the pyrochlore lattice has also been analyzed by using XPS and ESR spectroscopies and ac magnetic susceptibility measurements finding that the color observed for this material is due to Cr(IV) ions, most probably occupying the distorted octahedral position of Sn(IV)

Abstract: Pyrochlore zirconates are alternative materials to the presently used yttria-stabilized zirconia (7YSZ) for thermal barrier coatings. In this work, the use of europium as a solid-solution chromophore dopant is explored for measuring temperature in pyrochlore zirconates. Europium doping is found to give rise to strong luminescence as well as characteristic temperature-dependent luminescence lifetimes at high temperatures. Measurements of the luminescence lifetime of Eu-doped Gd 2 Zr 2 O 7 and Eu 2 Zr 2 O 7 pyrochlores, as well as Eu-doped YSZ, indicate sensitivity to temperature up to at least 1200 °C. The maximum temperature for Eu-doped Sm 2 Zr 2 O 7 is lower, about 1000 °C, for the dopant concentration used. These measurements provide the basis for non-contact sensing of temperature in europium-doped pyrochlores used in coatings.

Abstract: Investigations have been performed on the La-modified (1-x)Pb(Mg 1/3 Nb 2/3 )O 3 -(x)PbTiO 3 crystalline solution for compositions close to the morphotropic phase boundary (MPB) using energy-dispersive X-ray spectrometry (EDS). Studies were performed on specimens with La contents between 0 and 10 at.% for x=0.35 which were fabricated by keeping the average Mg/Nb/Ti ratio in the bulk ceramic unchanged. In this series of samples, La 3+ was compensated for by introducing B-site vacancies. Quantitative analysis by EDS revealed a change in the B-site cation concentrations with La substitution. It was observed that the Nb concentration decreases and the Ti concentration increases with increasing La content, indicating that local charge compensation in the perovskite phase is taking place by an adjustment in B-site cation concentrations. Phase analysis demonstrated the presence of a second phase for La contents above 2 at.%. Scanning electron microscopy then revealed the presence of a secondary pyrochlore phase with an octahedral-like morphology. The composition of the pyrochlore phase was found to be rich in Nb and poor in Ti. The effects of changes in composition on dielectric properties, polarization, and electrically induced strain were then investigated.

Abstract: Two beta-pyrochlore oxide superconductors, CsOs2O6 and RbOs2O6, are studied thermodynamically by measuring specific heat on polycrystalline samples. It is found that a Sommerfeld coefficient ? is nearly equal, 20 mJ/K2 mol Os, in the two oxides with different superconducting transition temperatures; Tc = 3.3 K and 6.3 K, respectively. This suggests that the density of states at the Fermi level is not a crucial parameter to determine the Tc of the beta-pyrochlore oxide superconductors, which is incompatible with the general expectation for a conventional BCS-type superconductor. Anomalous lattice contributions to specific heat at low temperature are also reported, which may come from nearly localized phonon modes associated with the rattling of the alkali metal ions weakly bound in an oversized cage formed by OsO6 octahedra.

Abstract: Pyrochlore-type (A 2 B 2 O 6 O') ceramics are considered for the immobilization of highly radioactive waste. Understanding the alteration process of such potential nuclear waste form materials in aqueous media is critical for the prediction of their long-term stability in a nuclear repository. Current models on pyrochlore alteration are based on a diffusion-controlled hydration and ion exchange process. However, we present results of a hydrothermal experiment at 200 °C with a natural, polycrystalline pyrochlore and 1 8 O-enriched aqueous solution, which are not compatible with a process based on solid-state diffusion. TOF-SIMS and confocal μ-Raman mapping of the run product revealed the occurrence of 1 8 O-enriched alteration zones with sharp chemical gradients to relict unreacted areas when compared to the extent of the alteration zones. The data are consistent with a pseudomorphic reaction that involves the dissolution of the pyrochlore parent accompanied by the simultaneous reprecipitation of a defect pyrochlore at a moving dissolution-reprecipitation front.

Abstract: The effects of niobium doping on the hysteresis parameters of sol-gel Pb-1.1-x/2(Zr0.53Ti0.47)(1-x)NbxO3 (0 < x < 0.05) have been reported fbr two sets of films with analogous grain size and degree of (111) texture but with different surface microstructures. For both sets, a strong continuous decay of the remnant and maximum polarizations and slope of the hysteresis loop at the coercive field was observed with increasing niobium concentration. The held dependence of the remnant polarization for any given niobium-doped him was identical to the functional field dependence of the undoped reference him, if multiplied by a niobium-concentration-dependent constant. Although the maximum and remnant polarizations decayed as the dopant level increased, their difference remained the same value as that of the undoped him at any given field. The width of the loop (at zero polarization) was insensitive to the niobium concentration at any given held, A linear increase in coercive held asymmetry (up to 40 kV/cm) was observed with niobium addition, yet was field independent and, thus, electrostatic in origin. Niobium governed switching through a reduction of the number of switching domains, without changing the total lattice polarization response. Microstructure-related effects on switching, such as decreased volume fraction of ferroelectric material or field-screening effects due to the presence of a pyrochlore second phase, were eliminated as the origin of the hysteresis changes. This paper has demonstrated how hysteresis features and their field dependencies can be used to separate the effects of niobium-induced microstructural changes from niobium lattice doping influences on the hysteresis loops.

Abstract: Behaviors of apparent phase changes of Ru-containing oxides in lead borosilicate glass at high temperature have been investigated using X-ray diffraction and transmission electron microscopy in application to thick-film resistors. During firing of thick films containing Ru oxide powder and lead borosilicate glass frits, apparent phase changes of Ru oxides have been found to occur both ways between ruthenium dioxide and lead ruthenate pyrochlore via decomposition of one phase in glass and subsequent formation of the other. The formation of pyrochlore occurs in a lead-rich form, Pb 2 (Ru 2-x Pb x )O 6.5 , whereas the formation of RuO 2 is characterized by a platelike morphology instead of initial globular morphology. A general tendency is observed that RuO 2 is stable in low-PbO glass compositions and at high temperatures, while Pb 2 (Ru 2-x Pb x )O 6.5 is stable in high-PbO glass compositions and at low temperatures, with the implication that the stability of these phases is dictated by the chemical activity of PbO in the glass melt.

Abstract: La2O3 doped nanocrystalline zirconia (ZrO2) has been prepared by chemical co-precipitation method for various dopant concentrations, varying from 3 to 30 mol%. Structural phases have been characterized by X-ray diffraction technique. All the as-synthesized samples were found to be in monoclinic phase. Annealing of the samples at different temperatures from 400 to 1000∘C stabilized ZrO2 either partially or fully the tetragonal/cubic phases. When they were annealed at 1200∘C, the monoclinic phase appeared again with a new cubic pyrochlore structured La2Zr2O7 at the expense of stabilized tetragonal phase. Formability of the tetragonal/cubic phase has been influenced by the dopant concentration and the annealing temperature. Sample with 8 mol% La2O3 has been stabilized completely in tetragonal/cubic phase after annealing at 900∘C for 1 h. Smallness of the grain in these nanocrystalline materials may also have assisted in the formation of La2O3-ZrO2 solid solution.

Abstract: The Bi2O3–Nb2O5–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi2O3, and the end-member of the δBi2O3–Nb2O5 solid solution. The existence of the previously reported Bi3Ni2NbO9 phase was disproved. A pyrochlore homogeneity range around Bi1.5Ni0.67Nb1.33O6.25 was determined together with all the phase relations in this phase diagram.

Abstract: Solid solutions in the Gd2(Ti1-yMoy)2O7 pyrochlore system, with y = 0.1, 0.3, 0.5, and 0.7, have been investigated as potential candidates for the anode material of solid-oxide fuel cells. The electrical conductivity, stability range, and optimal synthesis conditions of the pyrochlore phase have been explored by electrical, coulometric titration, and X-ray diffractometry techniques. As the molybdenum content increases, the maximum conductivity increases to a value of 70 S/cm for y = 0.7 at 1000°C, whereas the stability range in the oxygen partial pressure (pO2) decreases to just over two orders of magnitude in pO2 in the temperature range of 600°-1000°C. A decomposition reaction that leads to decreased conductivity in oxidizing atmospheres is proposed.

Abstract: Fluorite-related ceramics have attracted considerable attention as potential host materials for the immobilisation of radionuclides. Here we have used computer simulation to investigate the relative solution energies and mechanisms for Pu3+, Pu4+ and U4+ accommodation in an extensive range of A2B2O7 pyrochlore compounds. Solution is considered from simple oxides and via co-solution mechanisms. Results are discussed in terms of their implications for actinide retention and materials fabrication.

Abstract: The topotactic reduction of the pyrochlore Y2Ti2O7 with CaH2 is described. Neutron and X-ray powder diffraction data reveal the reduction yields mixtures of two reduced phases: Y2Ti2O6.48(2), a ma...

Abstract: Near normal incidence far infrared reflectivity spectra of [111] dysprosium titanate (Dy2Ti2O7) single crystal have been measured at different temperatures. Seven phonon modes (eight at low temperature) are identified at frequencies below 1000 cm−1. Optical conductivity spectra are obtained by fitting all the reflectivity spectra with the factorized form of the dielectric function. Both the Born effective charges and the static optical permittivity are found to increase with decreasing temperature. Moreover, phonon linewidth narrowing and a phonon mode shift with decreasing temperature are also observed, which may result from enhanced charge localization. The redshift of several low frequency modes is attributed to the spin–phonon coupling. All observed optical properties can be explained within the framework of the nearest neighbour ferromagnetic (FM) spin ice model.

Abstract: Antimonic acid Sb2O5·4H2O with the pyrochlore structure is a well known ion-exchanger that is selective for Sr2+ in mildly acidic solution. An enhancement of the ion exchange properties of antimony-based pyrochlore materials, with respect to Cs+ and Sr2+, was previously reported by Moller and co-workers to be achievable if Si4+ is incorporated during synthesis of these materials (T. Moller, R. Harjula, M. Pillinger, A. Dyer, J. Newton, E. Tusa, S. Amin, M. Webb and A. Araya, J. Mater. Chem., 2001, 11, 1526–1532). The present study aims to shed light on the role of Si4+ in the pyrochlore structure and its influence on the ion-exchange properties. A series of samples were prepared with increasing concentrations of Si added during preparation. X-Ray powder diffraction and electron microscope observations indicate that the average particle size of the antimony pyrochlores decreases as Si concentration increases. Refinement of X-ray powder data shows a small and abrupt decrease in unit cell volume at an added Si concentration corresponding to about 10 atom%. For undoped samples 29Si solid-state MAS NMR showed only a single sharp resonance at −75 ppm assigned to Si(OH)4 species located within the hexagonal channels of the pyrochlore structure. The increasing addition of Si to the reactant mixture gave rise to a progressive increase in the intensity of resonances from hydrated amorphous silicate species. Base treatment succeeded in removing only the resonances due to these silicate species, leaving the resonance assigned to structural Si unaffected. These data are taken as confirmation of the assignment of the sharp −75 ppm resonance to isolated species Si(OH)4 within the pyrochlore tunnels. Neutron powder diffraction of partially dehydrated samples shows the existence of two pyrochlore phases that appear to differ slightly in their unit cell parameters and confirms that incorporation of Si4+ into the pyrochlore structure and a reduction in water content cause volume contraction of the cubic cell. The slight unit cell contraction resulting from Si incorporation is hypothesised to be responsible for enhanced Cs+ selectivity of samples containing Si.