O7 Pyrochlore Research Articles

Low Temperature Microwave Properties of Bismuth Based Dielectric Ceramics

The microwave properties of Bi2(Zn1/3Nb2/3)2O7 and Bi2(Zn1/3Ta2/3)2O7 pyrochlore dielectric ceramics at low temperature were investigated. The ceramic samples were prepared by conventional mixed oxide technology. The structures of the samples were identified as monoclinic pyrochlores. The microwave properties of the cylindrical samples were measured of TE01δ modes at the temperature range of 10 K to 300 K in a cylindrical resonant cavity. The occupying of Nb5+ and Ta5+ in B sites lead different dielectric constants and low temperature dependencies of Q value. A low temperature dielectric anomaly in BZN was observed at 69 K which may be associated with a slight phase transition of the monoclinic pyrochlore Bi2(Zn1/3Nb2/3)2O7 polymorphs. The correlations between the structure, morphology and microwave dielectric properties were discussed.

Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd2(Ti1-yZry)2O7 Pyrochlores

Disorder in Gd2(Ti(1-y)Zry)2O7 pyrochlores, for y = 0.0-1.0, is investigated by Ti 2p and O 1s near-edge X-ray absorption fine structure spectroscopy. Ti(4+) ions are found to occupy octahedral sites in Gd2Ti2O7 with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.

Electrical Properties of Bi1.5ZnSb1.5O7 Pyrochlore Ceramics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electrical properties of Bi1.5ZnSb1.5O7 pyrochlore ceramics

AbstractBi1.5ZnSb1.5O7 pyrochlore samples were prepared by solid state reaction method. They were examined by x‐ray diffraction and scanning electron microscopy. Single phase, belongs to the cubic pyrochlore structure, with a lattice parameter of 10.442 Å and grain size that varies from 16 to 20 μm was obtained. The electrical properties were measured at different temperatures in the range 15–330 K under different applied magnetic fields up 1.4 T. In our measurements for Hall coefficient, Hall resistivity, and mobility; we noticed an anomalous behavior at two temperatures (around 250 and 310 K) which was supported by the I‐V measurements (double transition of the slope of I‐V characteristics (β) at the same temperatures). This was discussed in terms of polarization phenomenon and mixed ionic‐electronic conduction. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microwave Dielectric Properties of Bi2(Zn1/3Ta2/3)2O7 Polymorphs

Polymorphic phases of Bi2(Zn1/3Ta2/3)2O7 pyrochlore were studied. A metastable monoclinic structure was observed at temperatures below 950°C. This low-temperature phase possesses highly temperature-stable characteristics in the microwave frequency range (5–7 GHz) with a temperature coefficient of resonant frequency (τf)=-8.8 ppm/°C. The addition of B2O3 to Bi2(Zn1/3Ta2/3)2O7 significantly lowered the sintering temperature, to 850°C, without an associated structural transition. Low-temperature sintering mechanisms have been investigated using dilatometry and transmission electron microscopy. Bi2(Zn1/3Ta2/3)2O7 with 0.5 wt% B2O3 can be suitable for application in low-temperature cofired ceramics (LTCC), due to its reduced sintering temperature and good microwave dielectric properties: quality factor (Q×f)=3500 GHz at 6.7 GHz, dielectric constant (εr)=63.9 and τf=-14.1 ppm/°C.

Spectroscopic Investigations of the Structural Phase Transition in Gd2(Ti1-yZry)2O7 Pyrochlores

The Gd2(Ti1-yZry)2O7 pyrochlore series undergoes a structural phase transition from pyrochlore (Fd3m) to defect fluorite (Fm3m) that can be driven compositionally by increasing the Zr content or thermally by sintering Zr-rich compositions at temperatures above 1550 °C. Our results demonstrate that ion-beam irradiation can also drive the structural phase transition for Zr-rich compositions. In an effort to understand the effects of composition and ion-beam irradiation on this phase transition, powder X-ray diffraction, polarized Raman, reflection infrared, and X-ray absorption spectroscopy experiments were conducted on Gd2(Ti1-yZry)2O7 pyrochlores prior to and following irradiation with 2 MeV Au2+ ions to a fluence of 5 ions/nm2. Analysis of the vibrational and X-ray absorption data suggests that the structural integrity of the pyrochlore structure is based on distorted corner-shared TiO6 or ZrO6 octahedra. The vibrational spectra indicate that both anion and cation disorder precede the compositionally d...

Crystal Chemistry and Dielectric Properties of Chemically Substituted (Bi1.5Zn1.0Nb1.5)O7 and Bi2(Zn2/3Nb4/3)O7 Pyrochlores

The effect of the chemical substitution of Ti, Zr, and Gd cations on the structure and properties of bismuth‐based pyrochlores was investigated for (Bi1.5Zn1.0Nb1.5)O7 and Bi2(Zn2/3Nb4/3)O7. Broad ranges of solid solutions based on cubic (Bi1.5Zn1.0Nb1.5)O7 were observed for Ti and Zr cations: (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Zrx)O7 (0.0 ≤x≤ 1.5) and (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Tix)O7 (0.0 ≤x≤ 2.25). For Gd cations, the range of solid solution was quite limited in (Bi1.5−xGdxZn1.0Nb1.5)O7 (0.0 ≤x≤ 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi1.5−xGdxZn0.5Ti1.5Nb0.5)O7 (0.0 ≤x≤ 0.6)). The stability fields of the solid solutions could be interpreted by using the ratio of the ionic sizes of the A‐ and B‐site ions in the pyrochlore structure (RA/RB). The substituted phases exhibited permittivities (κ′) in the range of ∼100–200 and relatively low dielectric losses (tan δ < 7 × 10−4) at 1 MHz. The temperature coefficients of permittivity (τκ) varied in accordance with the RA/RB value and could be tuned from −88 ppm/K to −1300 ppm/K. For the orthorhombic Bi2(Zn2/3Nb4/3)O7 pyrochlore, the ranges of solid solution were very limited and the dielectric properties remained similar to those of the undoped phase (κ′= 90, τκ= 150 ppm/K).

Phase Stability and Electrical Conductivity in Gd2Ti2O7‐Gd2Mo2O7 Solid Solutions

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.

Rietveld X-ray Powder Profile Analysis and Electrical Conductivity of Fastion Conducting Gd2(Til-ySny)2O7 Solid Solutions

AbstractThe A3+2B4+2O7 pyrochlores have fast-ion conduction properties that make them attractive candidates for applications in fuel cells. Rietveld powder-profile analysis of x-ray diffraction data has been used to determine structural parameters for Gd2(Ti1-ySny)2O7 solid solutions (y = 0.20, 0.40, 0.60, and 0.80) for correlation with composition-dependence of the specimens' electrical conductivity. In accord with Vegard's law, the lattice constants increase linearly with y as the larger Sn ion replaces Ti4+. The structures in the system remain remarkably ordered in view of the fact that it had been suggested that increasing the average radius of the ions that occupy the B4+ site relative to A3+ drives the system toward a disordered defect-fluorite state. A small but significant increase in mixing of the occupancy of the cation sites was found with increasing y to a maximum of [GdB] = 0.05. We could detect no significant disorder in the anion array. The activation energy and pre-exponential term for oxygen ion conduction were found to increase linearly and exponentially with y, respectively, such that at a given temperature, the ionic conductivity changes by less than an order of magnitude across the system. The behavior is in marked contrast to other systems where the substitution of a larger cation in the B site increases conductivity by up to three orders of magnitude as a consequence of substantial disorder that is introduced in both the cation and anion arrays.

Fast-Ion Conducting Y2(ZryTi1-y)2O7 Pyrochlores: Neutron Rietveld Analysis of Disorder Induced by Zr Substitution

The structure of Y2(ZryTi1-y)2O7 solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A2B2O7□, space group Fd3m, to a deflect-fluorite structure (A, B)4 (O0.875□0.125)8, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-Å thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y = 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell—i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y ≤ 0.45 and is predominantly Y at y = 0.60. The substituted Zr4+ thus replaces Ti4+ in the six-coordinated B site for most of the solid-solution series, Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y ≤ 0.90. The disordered structural states cannot be described by a single-order parameter. The O(1) ion is displaced 0.46 Å toward the vacant O(3) site in the ordered pyrochlore at y = 0. The displacement relaxes to the ideal position of the fluorite structure with a quadratic dependence on y. Increase of the anisotropic temperature-factor coefficients with y indicates general softening of the structure with increased disorder. The array of O(1) ions forms a continuous path for migration of anions through the structure. The magnitude and composition dependence of the reported change of ionic conductivity with y may be satisfactorily explained by the variation of the product of charge-carrier concentration and vacancy concentration N(N - 1), where N is taken as the site occupancy of O(1).

Structural characteristics of pyrochlore formation

By calculating the value of the unknown oxygen positional parameter x for a large number of A3+2B4+2O7 pyrochlores, it was determined that increasing the radius of the A cation in a series of constant B increased the value of x, and that increasing the radius of the B cation in a series of constant A decreased the value of x. The region of pyrochlore formation has a range of x values from 0.404 to 0.432 and contains BO6 polyhedra that closely approach Oh symmetry and AO8 polyhedra that are highly deformed. It was shown that the characteristics of the BO6 polyhedra were the most important in determining whether or not a pyrochlore would form.