Search results for "Pyrochlore"
showing 10 items of 10 documents
Sol–Gel Synthesis of Tunable Cerium Titanate Materials
2008
Cerium titanate structures present a high technological interest because of their optical and catalytic properties. This work reports the synthesis of these materials by a sol–gel methodology that allows mixed oxides with TiIV/CeIII, TiIV/CeIV or TiIV/CeIII–CeIV species to be obtained. Crystallization of CeO2–TiO2 mixed oxides and Ce2Ti2O7 pyrochlore phase was corroborated by XRD and Raman spectroscopy. Magnetic and EPR measurements were performed in order to clarify the oxidation state of the cerium ions in the system as a result of the easy oxidation of CeIII. The firing atmosphere is crucial for the CeIII/CeIV ratio, which is responsible for the different structure, but it also affects t…
Conduction band polarization in some CMR materials
2000
First principles electronic structure calculations reveal certain important common features in the conduction band polarization of many of the newly examined colossal magnetoresistance (CMR) materials. Most CMR compounds seem to possess a localized, magnetic band slightly below the Fermi energy. This localized band transfers polarization to a relatively broad conduction band. The nature of the two bands in different systems can be quite distinct. In the perovskite-derived manganese oxides, the magnetic band is derived from $Mn t_{2g}$ states while the conduction band is derived from Mn e states. In the chalcospinel $Fe_{^0.^5}$ $Cu_{^0.^5}$ $Cr_{2}$$ S_{4}$ , the $Crt_{2g}$ states which are…
High pressure structure of Tb2Ti2O7 pyrochlore at cryogenic temperatures
2007
The structure of Th 2 Ti 2 O 7 pyrochlore was investigated at high pressures up to 24 GPa at cryogenic temperatures down to 6.5 K using angular dispersive X-ray diffraction with synchrotron radiation at HPCAT, Advanced Photon Source. The cell parameters were obtained by performing full profile Rietveld refinements of the diffraction data. The equation of state is obtained at low temperatures by fitting the pressure-volume data to a second order Birch Murnaghan eqation and a bulk modulus value of 168(4) GPa is obtained. The results show persistance of the pyrochlore structure up to the maximum pressure studied in the experiment and further indicate that pressure induces solely magnetic order…
On the existence of a pyrochlore-type phase in the system Bi2O3–TiO2
1995
The existence of a bismuth titanate in the range between Bi 2 Ti 4 O 11 (Bi 2 O 3 . 4 TiO 2 ) and Bi 4 Ti 3 O 12 (2 Bi 2 O 3 . 3 TiO 2 ) has been investigated by X-ray diffraction of samples prepared by solid state reactions. For reaction temperatures above 1100 °C and a starting composition Bi 2 O 3 . 2 TiO 2 there appeared additional lines which could be attributed to a cubic face-centred cell with a = 10.354 A. A multiphase Rietveld analysis based on X-ray powder diffraction data confirmed the structural model of a pyrochlore for this compound. There is evidence that this phase belongs to the group of defect pyrochlores with a Bi 3+ -deficiency resulting in a composition of Bi 1.833 Ti 2…
Processing and properties of nanocrystalline Pb(Sc0.5Ta0.5)O3, Pb(Sc0.5nb0.5)O3 and Pb(Mg1/3Nb2/3)O3 films produced by RF-sputtering from ceramic tar…
2000
Abstract Nanocrystalline thin films of different relaxor materials, namely Pb(Sc0.5Ta0.5)O3 (PST), Pb(Sc0.5Nb0.5)O3(PSN), Pb(Mg1/3Nb2/3)O3(PMN) have been produced by RF-sputtering to investigate whether it will affect their dielectric properties if their grain size is reduced to the dimensions known from their nanodomains. The XRD shows that the amorphous film crystallizes in pyrochlore structure at lower temperatures and short times. Annealing at higher temperatures and far longer time intervals leads to an increasing amount of perovskite phase with a grain size in the nanometer range. These results including dielectric measurements will be presented and discussed.
ELECTRICAL PROPERTIES OF LEAD FERROTANTALATE CERAMICS
2009
ABSTRACT Perovskite structures with high dielectric constants and magnetic properties play an important role in micro- and optoelectronics and have numerous practical applications. Relaxor type ferroelectric ceramics PbFe1/2Ta1/2O3 with perovskite structure was subject of present studies. Pyrochlore free lead ferrotantalate ceramics were produced by solid state technology from oxides. Low- and infra-low frequency studies of dielectric properties of PbFe1/2Ta1/2O3 ceramics have revealed considerable infra-low frequency dispersion at temperatures above the temperature of maximum dielectric permittivity. The observed dispersion described by lemniscates is due to a considerable Maxwell-Wagner r…
Facile hydrothermal synthesis of crystalline Ta2O5 nanorods, MTaO3 (M = H, Na, K, Rb) nanoparticles, and their photocatalytic behaviour
2014
Alkali metal tantalates are of interest for applications in photocatalysis as well as in high temperature resistance or capacitor dielectric materials. We have synthesized nanosized Ta2O5 rods and MTaO3 cubes (M = Na, K, Rb) hydrothermally and demonstrate the pH dependence of the synthesis of tantalum oxide and tantalate nanoparticles. The morphologies of the nanoparticles range from particle agglomerates in acidic reaction media over rods at neutral pH to tantalate cubes in basic reaction media. Whereas there is no apparent influence of the base cation on the particle morphology, there is a pronounced effect on the particle composition. At high base concentrations cubic tantalate particles…
The Effect of Technological Factors on the Synthesis of PbMg 1/3 Nb 2/3 O 3
2003
Conventional technique of solid phase reactions allows one to obtain PbMg 1/3 Nb 2/3 O 3 material with about 20 vol.% of the pyrochlore phase. The latter can be reduced to 5 vol.% by Swartz-Shrout two-stage solid state reaction via MgNb 2 O 6 having the columbite structure. A single-phase perovskite product can be obtained by by the two-stage columbite method from mixtures containing 0.2 mol. parts of overstoichiometric MgO. The best results are obtained by using magnesium carbonate hydroxide pentahydrate (MgCO 3 ) 4 ·Mg(OH) 2 ·5H 2 O as the magnesium source. The modified route yields stoichiometric single-phase PMN.
Mössbauer study of properties and phase transitions of Cd2Nb2–2xSn2xO7–2xF2x
1973
With the aid of Mossbauer spectroscopy the solid solution series Cd2Nb2–2xSn2xO7–2xF2x are investigated in the temperature range 4.2 to 400 K. The crystal structure belongs to the cubic pyrochlore type. The BY6 octahedra (B = Nb, Sn; Y = O, F) are linked three-dimensionally through their corners, forming groups of four octahedra which are extended or compressed in the (111)-direction for x ≤ 0.375 or x ≥ 0.400 respectively. The dependence of the quadrupole splitting ΔEQ on x shows a small but significant discontinuity in the region 0.375 < x < 0.400. This can be attributed to a first order phase-transition which depends on concentration, but not on temperature over the temperature region st…
Phonon properties of the spinel oxideMgTi2O4with theS=1/2pyrochlore lattice
2003
We study the phonon dynamics of ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ spinel by measuring the Raman and infrared reflectivity spectra in a wide frequency $(100--3000{\mathrm{cm}}^{\ensuremath{-}1})$ and temperature (10 K--300 K) range. The reflectivity spectra are analyzed by a fitting procedure based on a model which includes both Drude and phonon oscillator contributions to the dielectric constant. The phonon assignment is done from comparison between experimental data and shell model lattice dynamical calculations. We find two infrared-active ${F}_{1u}$ symmetry modes superimposed on the free carrier continuum, and four Raman-active modes of ${1A}_{1g},$ ${1E}_{g},$ and ${2F}_{2g}$ symme…