Abstract: A novel macroscopically degenerate state called kagome ice, which was recently found in a spin ice compound Dy 2 Ti 2 O 7 in a magnetic field applied along the [111] direction of the cubic unit cel

Abstract: Novel composite materials have been developed as alternative forms to immobilise nuclear solid waste. These composites are made of a lead-containing glass matrix, into which particles of lanthanum zirconate pyrochlore are embedded in 10 and 30 vol% concentrations. The fabrication involves powder mixing, pressing and pressureless sintering. The processing conditions were investigated with the aim of achieving the highest possible density. The best composites obtained showed a good distribution of the lanthanum zirconate particles in the glass matrix, strong bonding of the particles to the matrix and relatively low porosity (<10%). The best sintering temperature was 600°C for the 10 vol% composite and 650°C for 30 vol%. Sintering was carried out for an hour and a heating rate of 10°C · min−1 was shown to be superior to a heating rate of 2°C · min−1. At the relatively low sintering temperatures used, the pyrochlore crystalline structure of lanthanum zirconate, relevant for containment of radioactive nuclei, was stable.

Abstract: We have studied the growth of epitaxial La2Zr2O7 thin films on (111) Si. Although the interface structure can be strongly affected by the Si oxidation during the deposition process, epitaxial growth of La2Zr2O7 was obtained. A detailed study by means of transmission electron microscopy reveals two types of structures (pyrochlore and fluorite) with the same average chemical composition but strong differences in reactivity and interface formation. The structural complexity of the ordered pyrochlore structure seems to prevent excess oxygen diffusion and interfacial SiO2 formation.

Abstract: Well-defined Ln2Sn2O7 powders (Ln = La, Sm and Gd) with a phase-pure pyrochlore structure were synthesized by hydrothermal reaction. The catalytic activities of Ln2Sn2O7 powders for methane combustion were measured. Methane oxidation started at 500 °C and increased with oxidation temperature. Catalytic methane combustion is strongly influenced by the presence of oxygen vacancies that form by breaking Sn–O lattice bonds as the temperature increases. Addition of manganese to the rare earth pyrochlores improved methane oxidation activity. Manganese-doped samarium stannate pyrochlore (Sm2Sn1.8Mn0.2O7) shows highest the catalytic activity. Light-off and complete oxidation temperatures were measured at about 400 and 650 °C, respectively.

Abstract: Thermodynamic modeling was used to predict the optimum synthesis conditions for precipitation of the phase-pure lead titanate (PbTiO 3 ) in the Pb–Ti-tetramethylammonium hydroxide (TMAH) system using a newly developed computer program for automatic generation of stability and yield diagrams. The thermodynamic model has been experimentally validated over a wide range of processing conditions. Like KOH-mineralized systems, it was determined that the pH of the hydrothermal reaction medium and the Pb/Ti ratio are critical factors in forming stoichiometric PbTiO 3 powder. Morphological evolution during the reaction suggests that the formation mechanism appears to be controlled by a dissolution and recrystallization process. Two possible growth mechanisms are proposed based on the magnitude of the Pb/Ti ratio. In the case of Pb/Ti ratio=1.1, at the early stage of the reaction (3 h) excess lead species promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PbTiO 3 crystals. The growth of cubic PbTiO 3 crystals proceeds until the intermediate phase acting as a reservoir to provide precipitating ions is consumed. In case of Pb/Ti ratio=1.25, excess lead condition leads to the formation of a platelet-shaped intermediate pyrochlore phase. These platelet intermediate particles act as a template in which small cubic shaped PbTiO 3 grains grew on the surface of these platelets.

Abstract: A coating approach for synthesizing 09Pb(Mg 1/3 Nb 2/3 )O 3 -01PbTiO 3 (09PMN-01PT) and PMN using a single calcination step was demonstrated The pyrochlore phase was prevented by coating Mg(OH) 2 on Nb 2 O 5 particles Coating of Mg(OH) 2 on Nb 2 O 5 was done by precipitating Mg(OH) 2 in an aqueous Nb 2 O 5 suspension at pH 10 The coating was confirmed using optical micrographs and zeta-potential measurements A single calcination treatment of the Mg(OH) 2 -coated Nb 2 O 5 particles mixed with appropriate amounts of PbO and PbTiO 3 powders at 900°C for 2 h produced pyrochlore-free perovskite 09PMN-01PT and PMN powders The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb 2 O 5 by the Mg(OH) 2 coating The Mg(OH) 2 coating on the Nb 2 O 5 improved the mixing of Mg(OH) 2 and Nb 2 O 5 and decreased the temperature for complete columbite conversion to ∼850°C The pyrochlore-free perovskite 09PMN-01PT powders were sintered to 97% density at 1150°C The sintered 09PMN-01PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz

Abstract: Bi1.5ZnSb1.5O7 dielectric ceramic with pyrochlore structure was investigated by impedance spectroscopy from 400 to 750 °C. Pyrochlore was synthesized by the polymeric precursor method, a chemical synthesis route derived from Pechini’s method. The grain or bulk resistance exhibits a sensor temperature characteristic, being a thermistor with a negative temperature coefficient (NTC). Only a single region was identified on the resistance curve investigated. The NTC thermistor characteristic parameter (β) is equal to 7140 °C, in the temperature range investigated. The temperature coefficient of the resistance (α) was derived, being equal to −4.46×10−2 °C−1 at 400 °C. The conduction mechanism and relaxation are discussed.

Abstract: The cubic pyrochlore structure of Bi 1.5 Zn 1.0 Nb 1.5 O 7 was investigated. The crystal chemical formula of this material was determined as Bi 1.5 Zn 0.5 (Zn 0.5 Nb 1.5 )O 7 by comparing the calculated theoretical X-ray diffraction intensities with the observed ones. Crystal structure parameters were refined by Rietveld method. The (Bi 1.5−2 y Zn 1+2 y Nb 1.5 )O 7− y crystalline samples were also examined by Raman spectroscopy to look for further short range structural information. The results show that the Nb–O and Zn–O stretching modes in the B site oxide octahedral almost do not change with the chemical constitution changing. It is reasonable that the movable Zn 2+ is inclined to occupy B site first and then enter into A site after the B site was stuffed.

Abstract: The chemical interactions between porous (La 0.8 Sr 0.2 )MnO 3 (LSM) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate have been investigated over the temperature range of 1300–1500 °C in air. Two distinct reaction layers of fluorite-type cubic zirconia solid solution c-(Zr,Mn,La,Y)O 2 and lanthanum zirconate pyrochlore (La,Sr) 2 (Zr,Y) 2 O 7 were observed at the interface of LSM/TZ3Y. It has been found that the diffusion/dissolution of Mn ions in TZ3Y leads to the formation of the fluorite-type cubic zirconia solid solution, while the interaction of lanthanum with TZ3Y results in the formation of the lanthanum zirconate pyrochlore phase. Phase studies in the (ZrY)O 2 –La 2 O 3 –Mn 3 O 4 system show that the fluorite-type cubic zirconia solid solution phase c-(Zr,Mn,La,Y)O 2 , rather than the tetragonal 3 mol% Y 2 O 3 –ZrO 2 phase, is in equilibrium with LSM perovskite at high temperatures. A ternary phase diagram of the system at the (ZrY)O 2 -rich end at 1400 °C in air was proposed based on the experimental results. It is suggested that the fundamental reason for the beneficial effect of A-site non-stoichiometry or Mn excess of LSM in the inhibiting of the lanthanum zirconate formation is due to the fact that Mn 3 O 4 does not equilibrate with lanthanum zirconate at high temperatures.

Abstract: The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7$ (at 1K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency; furthermore, no long range order is observed. These unusual features we attribute to frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.

Abstract: The structural and compositional evolution of four members of the ANbWO6 (A = NH4+, Rb+, H+, K+) defect pyrochlore family have been studied as a function of pressure up to 7 GPa, using a diamond an...

Abstract: Electronic properties of the metallic pyrochlore oxide Cd 2 Re 2 O 7 are studied by means of electrical resistivity and Hall measurements. Semimetallic band structures are revealed as expected from band structure calculations. It is found that large changes in carrier density and mass occur at the structural phase transition at T s1 =200 K. A large mass enhancement is observed, particularly for the high-temperature phase with the ideal pyrochlore structure, suggesting that an anomalous correlation has an important effect on the itinerant electrons in the pyrochlore lattice.

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Abstract: Modification of dielectric characteristics of Bi1.5ZnNb1.5O7 pyrochlore was performed by Sr substitution for Zn in A-site together with the phase evolution. In general, the ceramics prepared by solid-state reaction are multiphase materials. However, single phase specimens having cubic pyrochlore structure could be obtained in the vicinity of Sr=0.1. The temperature coefficient of the dielectric constant (αc) of Bi1.5ZnNb1.5O7 increased remarkably by Sr substitution, but dielectric constant varied slightly while the dielectric loss increased steadily. Ceramics of those series have dielectric properties of high e (>145) and low temperature coefficient. The A-site cation substitution caused a very slight change in dielectric constant, suggesting little effect of the substitution on the ionic polarization of the pyrochlore structure.

Abstract: Systemic syntheses showed that the lead titanate pyrochlore phases formed under hydrothermal reactions in a certain range of KOH concentrations. The obtained pyrochlore phases all crystallize in the cubic structure. In comparison with typical pyrochlore structure, the Osite in Pb2Ti2O6Ois vacant. In addition, considerable Pb vacancies are present in the samples obtained with high KOH content. Pb(IV) is present in these samples as confirmed by structural and spectroscopic studies. The distribution of metal atoms (Pb and Ti) in pyrochlore phases depends crucially on the KOH content in the starting materials. Higher KOH content tends to form ordered structures where Ti and Pb occupy 16c and 16d positions, respectively. While in the lower KOH content samples, Ti and Pb tend to be randomly distributed within these two sites. The morphology of the pyrochlore samples obtained by hydrothermal synthesis is uniformed spherical aggregates, which are formed by sparsely jointed nanoparticles. Thus, these materials contain narrowly distributed cavities and exhibit rather high surface area. The lead titanate pyrochlore is not stable at high temperature and transforms to the perovskite phase above 651 °C.

Abstract: This paper reviews the recent developments in the understanding of the radiation-damage processes in A2B2O7 (Fd3m; Z=8) pyrochlore-structure compounds Pyrochlore structure compounds display a wide range of behaviors in response to ion beam irradiation Some compositions, such as Gd2Ti2O7, are amorphized at relatively low doses (~02 dpa at room temperature) while other compositions, such as Gd2Zr2O7, do not amorphize (even at doses of 36 dpa at 25 K) and instead disorder to a defect fluorite structure The response to ion beam irradiation is highly dependent on compositional changes that affect both the structural distortion from the ideal fluorite structure and the associated energetics of the disordering process Generally, the ionic size of the cations plays a dominant role in determining the radiation response of different pyrochlore compositions However, the cation ionic radius ratio criteria cannot be applied all-inclusively in predicting the radiation “tolerance” of a pyrochlore Systematic irradiation studies of the radiation response of rare-earth (A-site) pyrochlores in which B = Ti, Zr, and Sn have shown that the behavior of the pyrochlore also depends on the cation electronic structure, ie, the type of bonding, which is closely related to the polyhedral distortion and structural deviation from the ideal fluorite structure These structural changes affect the dynamic defect recovery process directly linked to the material's response to and recovery from irradiation

Abstract: Sol-gel derived (Pb,La)(Zr,Sn,Ti)O3 powders and sintered ceramics were prepared; their crystallization kinetics and dielectric properties were studied as a function of temperature. X-ray powder diffraction showed that the pyrochlore phase formed initially from an amorphous gel during low-temperature heat treatment around 200–300 °C. Further heat treatment to 600 °C resulted in the development of the perovskite phase with no significant growth of pyrochlore crystallites. At intermediate temperatures the sol-gel derived powders yielded a mixture of pyrochlore and perovskite phases. The crystallization kinetics of the perovskite phase was analyzed following the Avrami approach, which indicated that the process is diffusion limited. Those fine powders were utilized to prepare compact ceramics after sintering at various temperatures; their dielectric properties were analyzed. The electric field induced transformation from the antiferroelectric to the ferroelectric phase was clearly observed through the polariz...

Abstract: Neutron diffraction studies have been carried out in the applied magnetic field H (∥[0\bar11]) on a single crystal of pyrochlore ferromagnet Nd 2 Mo 2 O 7 , whose Hall resistivity (ρ H ) has been reported to have quite unusual magnetic field ( H )- and temperature ( T )-dependences. The intensities of the observed magnetic reflections have been reproduced at 1.6 K as a function of H , by considering the change of the magnetic structure with H , where effects of the exchange fields at the Mo and Nd sites induced by the Mo–Mo and Mo–Nd exchange interactions and the single ion anisotropies of Mo- and Nd-moments are considered. From the H -dependent magnetic structure, the H -dependence of ρ H has been calculated by using the chiral order mechanism. By comparing the result with the H -dependence of the observed ρ H , it is found that the chiral order mechanism does not work well in the present system.

Abstract: Transmission electron microscopy-selected area electron diffraction studies were performed on as-pyrolyzed sol-gel-derived Pb(Zr1-x, Tix)O3 films deposited on Pt electrode layers to elucidate the structural cause behind the large effects of pyrolysis conditions on orientation selection [L. Fe, G.J. Norga, H.E. Maes, and G. Maes, J. Mater. Res. 16, 2499 (2001)]. The crystallinity of the intermediate pyrochlore phase, which forms during pyrolysis, was found to strongly depend on pyrolysis conditions. Specifically, pyrolysis for 10 s at 350 °C was seen to result in conversion of the film to a well-crystallized, oxygen-deficient pyrochlore phase with the fluorite crystal structure (disordered pyrochlore). We speculate that formation of the metastable fluorite phase is favored by the reduced oxygen partial pressure, caused by burnoff of residual acetates. Longer pyrolysis times and/or higher pyrolysis temperatures result in a quasi-amorphous intermediate phase. The presence of the well-crystallized fluorite phase in the pyrolyzed film is seen to result in (111)-oriented films after crystallization, while pyrolyzed films consisting of a quasi-amorphous intermediate phase turn out (100) or mixed (100)/(111) oriented after crystallization. An explanation for the observed orientation effects, on the basis of the different surface energetics of fluorite versus perovskite structure oxides, is proposed.

Abstract: We have made detailed studies of pyrochlore compounds, R 2 Mo 2 O 7 with R = Nd , Sm , Gd , and Dy. For R=Nd and Sm, we found that apart from the ferromagnetic transition of Mo moments there is another transition of a reentrant spin glass type at lower temperatures, where both the magnetization and heat capacity data show anomalous features. We also observed that the AC susceptibility data are clearly frequency-dependent even in the ferromagnetic phase. In our pressure-dependent magnetization studies, the magnetization data of (Gd 1− x Dy x ) 2 Mo 2 O 7 for x=0, 0.1, 0.2 , and 0.4 are found to decrease continuously with pressures and, simultaneously, the ferromagnetic transition temperatures move towards lower temperatures. Eventually the ferromagnetic state of (Gd,Dy) 2 Mo 2 O 7 becomes unstable at higher pressures, and gives way to a spin glass phase that is found to exist at lower temperatures.

Abstract: Perovskite developments in the Pb(Zn1/3Ta2/3)O3–PbTiO3 system were explored. Formation yields and lattice parameters of the perovskite were determined from X-ray diffractometry results. Weak-field low-frequency dielectric properties of the system ceramics were investigated, followed by microstructure examination. Perovskite started to develop in Pb(Zn1/3Ta2/3)O3 after the introduction of 30 mol% PbTiO3, whereas complete stabilization was accomplished at 60% substitution. Dielectric relaxation behavior was not substantial across the entire composition range, whereas phase transition modes changed from diffuse to sharp with increased PbTiO3 fraction.

Abstract: This letter reports the dielectric properties of the cubic pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7, monoclinic zirconolite Bi2(Zn1/3Ta2/3)2O7, and their composites in the temperature range of 10–400 K from 102 to ∼1010 Hz. Composites with a near zero temperature coefficient of capacitance (TCC) consisting of the cubic pyrochlore and monoclinic zirconolite phases have been obtained. The observed microwave dielectric properties (e=∼60, Q=∼325, and TCC=∼30 ppm/K at ∼5.1 GHz) as well as their low sintering temperatures (950–1100 °C) show that the compounds are promising materials for recently developed microwave devices, which demand that dielectric ceramics can be cofired with silver electrodes.

Abstract: Nominal Pb2FeReO6 adopts a defect pyrochlore structure at ambient pressure unlike the other A2FeReO6 (A = Ca, Sr, Ba). Rietveld refinement of the crystal structure of one of the compositions Pb2FeReO6.1 from powder XRD data shows that the structure is cubic pyrochlore (a = 10.382 A; space group: Fd3m) where oxygen vacancies occur at O2 (8b) sites. The divergence between the ZFC and FC magnetic susceptibility data and the non-Arrhenius resistivity behaviour of Pb2FeReO6 compositions are characteristic of the underlying geometrically frustrated Fe/Re cation sublattice in the pyrochlore structure.