Defect Pyrochlore Research Articles

Synthesis and structural characterization of new defect pyrochlore type A xLn ySb 2O 6+ z antimoniates (0< x, y, z

New cubic defect pyrochlore type A x Ln y Sb 2O 6+ z antimoniates (0< x, y, z<1; A=Rb, Cs, Tl; Ln=Eu, Gd, and Y) (space group Fd3 m) were prepared by calcination in air at 650 °C of compositions obtained by a coprecipitation method. Their crystal structures were refined by the Rietveld procedure and further confirmed by X-ray absorption spectroscopy (EXAFS) experiments. Refinements of the X-ray powder diffraction data show a partial and statistical occupation of the 8b site by alkaline or thallous ions and oxygen and lead to values close to 0.33 for the only refinable positional x parameter of the 48f oxygenated site.

Pressure-induced cation migration and volume expansion in the defect pyrochlores ANbWO6 (A = NH4+, Rb+, H+, K+).

The structural and compositional evolution of four members of the ANbWO(6) (A = NH4+, Rb+, H+, K+) defect pyrochlore family have been studied as a function of pressure up to 7 GPa, using a diamond anvil cell and monochromatic synchrotron X-ray powder diffraction. In response to increasing hydrostatic pressure, NH(4)NbWO(6) and RbNbWO(6) both initially contract but then undergo a fairly abrupt increase in their unit cell volumes above a characteristic threshold pressure. NH(4)NbWO(6) exhibits a 5.8% increase in the cubic unit cell edge once the pressure exceeds approximately 3.4 GPa, while the RbNbWO(6) unit cell expansion is larger ( approximately 7.5%) but less abrupt, beginning near 3.0 GPa. Rietveld refinements reveal that the reversible expansion is driven by insertion of water into the structural channels that interpenetrate the NbWO(6)(-) octahedral corner sharing framework. The insertion of extra water is accompanied by displacement of the NH(4)(+) or Rb(+) ions to a smaller site in the channel structure, which triggers the pressure-induced expansion of the pyrochlore framework. This mechanism explains the counterintuitive expansion of the pyrochlore framework in response to application of external pressure. It should be noted that the expansion exhibited by the pyrochlore framework must coincide with a decrease in the volume of the hydrostatic fluid so that the net volume of the system decreases with increasing pressure. Similar behavior is not observed for KNbWO(6).H(2)O or HNbWO(6).H(2)O, both of which contract in response to increasing pressure. For these smaller monovalent cations, pressure-induced volume expansion does not occur because the hydrated state and subsequent cation shift are already stable at ambient conditions.

Ion-conducting defect pyrochlore phases in the K2O-Sb2O3-WO3 system

Potassium tungstate antimonates were prepared by calcining αK2CO3 + βSb2O3 + 2(1 – α – β)WO3 mixtures in air. Data on the phase composition of the obtained materials were used to locate the pyrochlore phase region in the Sb2Oz–W2O6 – K2O composition triangle at 1170 K. The distributions of the constituent ions and lattice defects over the crystallographic positions of space group Fd3m were inferred from structural and gravimetric data. The ac conductivity of the potassium tungstate antimonates was measured from 600 to 1000 K. The conductivity and its activation energy were shown to be correlated with the concentrations of cation (position 16d) and anion (position 8b) defects. The concentration and mobility of K+ ions involved in charge transport were determined.

Pb2FeReO6: new defect pyrochlore oxide with a geometrically frustrated Fe/Re sublatticeElectronic supplementary information (ESI) available: powder XRD patterns of pyrochlores Pb2FeReO6.1 and Pb2FeReO5.81. See http://www.rsc.org/suppdata/jm/b3/b304118m/

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.

Hydrothermal synthesis of perovskite and pyrochlore powders of potassium tantalate

Potassium tantalate powders were hydrothermally synthesized at 100 to 200 °C in 4 to 15 M aqueous KOH solutions. A defect pyrochlore, Kta2O5(OH).nH2O (n≈ 1.4), was obtained at 4 M KOH, but at 7–12 M KOH, this pyrochlore was gradually replaced by a defect perovskite as the stable phase. At 15 M KOH, there was no intermediate pyrochlore, only a defect perovskite,0.85Ta0.92O2.43(OH)0.570.15H2O. Synthesis at higher KOH concentrations led to greater incorporation of protons in the perovskite structures. The potassium vacancies required for charge compensation of incorporated protons could accommodate water molecules in the perovskite structure.

Hydrothermal epitaxy of KtaO3 thin films

Heteroepitaxial KTaO3 thin films were synthesized by the hydrothermal method below 175 °C in highly alkaline aqueous KOH solutions. KTaO3 films synthesized in 7 M KOH solutions contained a defect pyrochlore phase that had (400) and (111) out-of-plane orientations. The (400) oriented pyrochlore islands were epitaxially related to the (100) SrTiO3 substrate while the (111) oriented pyrochlore islands had four in-plane variants. Pyrochlore-free KTaO3 films could be synthesized in a 15 M KOH solution. Pyrochlore-free films were also grown at 7 M KOH by delaying the introduction of the (100) SrTiO3 substrate into the synthesis solution until the conditions favoring the formation of the pyrochlore phase had dissipated.

ChemInform Abstract: Synthesis and Structural Characterization of the New Defect Pyrochlore Nd1.68Ti2O6.52.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis and structural characterization of the new defect pyrochlore Nd1.68Ti2O6.52

Abstract The new defect pyrochlore phase, Nd1.68Ti2O6.52, has been prepared in air at 800 °C starting from precursor materials obtained via a ‘liquid mix’ process. It crystallizes in space group Fd 3 m of the cubic system with Z=8 and a=10.233(3) A. Its structure was determined from a Rietveld analysis. The outstanding feature of the structure is the 7-fold oxygen coordination of the Nd atom (six 48f O atoms and one 8b O′ atom) which enables to reach a bond valence sum close to +3 for Nd. EDX analyses on many crystals picked up from different preparations have shown that the Nd content of the defect pyrochlore Nd2–x[Ti2O6]O′1–3x/2 always falls within a very narrow range. This result is in very good agreement with the theoretical composition range that can be inferred from structural considerations taking into account the 7-fold oxygen coordination of Nd as a requirement. HREM investigations and image calculations confirm the defect character of the structure.

The Structural Transformation from the Pyrochlore Structure, A2B2O7, to the Fluorite Structure, AO2, Studied by Raman Spectroscopy and Defect Chemistry Modeling

Pyrochlore oxides of the composition Y2Ti2−yZryO7, with y= 0, 0.3, 0.6, and 0.9, were studied using Raman spectroscopy. Doping with Zr+4, a homovalent ion, gradually induces the material to undergo a structural change from the perfect pyrochlore structure to a defect pyrochlore structure and ends at an oxygen deficient fluorite structure with the formula (Y, Ti, Zr)1O1.75. Invoking the bigger Zr+4 ion on the Ti+4 site causes this structural transformation. We have also investigated highly Ti-doped YSZ ceramic samples, which all have an oxygen deficient cubic fluorite structure. These samples have formula units between (Y, Ti, Zr)1O1.675 and (Y, Ti, Zr)1O1.90. It is argued that a band observed at 750 cm−1 in both the Ti-doped pyrochlore samples and in the Ti, Y, Zr fluorite samples is due to oxygen in seven fold coordination around Ti. Local structures are accordingly observed by Raman in these samples. We finally report a Raman spectrum of orthorhombic Y2TiO5. A defect chemistry model has been formulated, which qualitatively describes the observed pyrochlore–fluorite structural transition.

The catalytic effects of the Mg 2+-doping on the ethanol-sensing properties of Cd 2Sb 2O 6.8

In this paper, the ultramicro powders of the solid solution (Cd 1− x Mg x ) 2Sb 2O 6.8, with defect pyrochlore structure, were prepared by a chemical cocoprecipitation method. The phase constituents of the powders were characterized by X-ray diffraction (XRD), and the results revealed that the substitutional solid solutions formed when 0≤ x<0.20, and that Mg 2+-addition could inhibit the growth of the crystalline grains of the product powders during firing treatment process. The influences of Mg 2+-addition on ethanol-sensing properties of the system were also investigated. It was found that Mg 2+-doping greatly improves the sensitivity, selectivity, stability, response and recovery characteristics of the material to ethanol, and decreases the operating temperature from 330 to 250°C. All of these effects might due to the catalytic characteristics of Mg 2+-doping.

Nouveaux matériaux ABi 2B 5O 16 (A = Na, Ag, K, Rb, Tl et B = Nb, Ta) de type pyrochlore déficitaire

New defect pyrochlore oxides ABi 2B 5O 16 (A = Na, Ag, K, Rb, Tl and B = Nb, Ta) have been isolated in the pseudo-binary systems BiBO 4 — AB 3O 8. The crystal structures of six of these oxides with respective compositions ABi 2B 5O 16 (A = K, Tl and B = Nb, Ta) and ABi 2Ta 5O 16 (A = Na, Ag) have been determinated by Rietveld profile refinement of their X-ray diffraction powder patterns. According to the size of the monovalent A + cations, two different defect pyrochlore structures have been found. When the A + cations are Na + and Ag +, the occupied sites are those of the ideal pyrochlore structure: (A,Bi) on 16d site, Ta on 16c site, O on 48f site and O' on 8b site. When the A + cations are bigger, Tl + or K + cations, the structure is of the type proposed for the homologous compounds K 0.51Sb 0.67(Sb 2O 6)O' 0.26 [1], Tl 0.51Sb 0.71(Sb 2O 6)O' 0.32 [2] and A 0.4Bi 0.8(B 2O 6)O' 0.4 (A = Rb, Cs and B = Nb, Ta) [3] phases: Bi on 96g site, B (Nb, Ta) on 16c site, O on 48f site, O' on 8b site and A on 32e site.

A study on the novel gas-sensing solid solution material (Cd 1− xAg x) 2Sb 2O 6.8

In this paper, the ultrafine powders of the solid solution (Cd 1− x Ag x ) 2Sb 2O 6.8 were prepared by a chemical coprecipitation process. The powders were proved by X-ray diffraction (XRD) to bear a similar defect pyrochlore structure to Cd 2Sb 2O 6.8 when x≤10 mol%. The results show that the doped silver ion affects the conductance as well as the gas-sensing properties, and the conductance decreases steadily with the increasing silver content while the sensitivity of the sample with x=0.05 to 1000 ppm C 2H 5OH is about five times that of the pure Cd 2Sb 2O 6.8. Meanwhile, the doping lifted the selectivity toward gasoline and reduced the optimum operating temperature. All of these suggest this solid solution as a candidate for practical alcohol sensors.

Observations on pyrochlore oxide structures

The structures of three defect pyrochlore oxides were refined using powder neutron or synchrotron X-ray diffraction data: Bi 1.89GaSbO 6.84 ( Fd3̄ m; a = 10.3807(4) Å); Pb 2(Ga 0.5Sb 1.5)O 6.5 and Pb 2(Ni 0.333Sb 1.667)O 6.5 ( F4̄3 m; a = 10.4433(2) and 10.5052(2) Å, respectively). Based on these and previously reported structures, some generalizations on the structure of defect pyrochlores are made. It was found that the contraction of the lattice in metallic Bi 2B 2O 7−y-type oxides, relative to that of the isostructural insulating oxides, is not a result of B–B bonding. Rather, there is a contraction of the BiO 8 polyhedron in the metallic oxides, due to the decreased stereochemical activity of Bi 6s electrons, as a consequence of their mixing with B d orbitals. In A 2B 2O 6.5 compounds, where A is an easily polarizable cation, oxygen-vacancy ordering accompanied by A-cation displacement can occur. The extent of the displacement of the A-type cation is shown to be strongly dependent on the cell parameter.

Study on conduction mechanism and gas-sensing property of semiconducting material Cd 2Sb 2O 6.8

In this paper, the pure ultramicro powders of Cd 2Sb 2O 6.8 with a defect pyrochlore structure have been prepared by a chemical coprecipitation method for the first time. The conduction mechanism and gas-sensing properties of the pure powders were studied. The results show that Cd 2Sb 2O 6.8 is a n-type semiconductive material and its gas-sensing mechanism is of the surface resistance control type; the element made of Cd 2Sb 2O 6.8 powder is sensitive to several gases such as C 2H 2, TMA, LPG and acetone, and exhibits higher sensitivity and better selectivity to C 2H 2 especially. Thus, this complex oxide is a promising acetylene-sensing material.

Oxygen vacancy control in the defect pyrochlore: a way to tune the electronic bandwidth

An oxygen deficient pyrochlore has been prepared by heating mixtures of and at in air and a subsequent treatment under Ar atmosphere. This material is metallic between 4 and 290 K, showing an increased resistivity, by one order of magnitude at 300 K, with respect to stoichiometric . The structure is cubic, a = 10.2991(7) Å, and can be described in the space group . A profile refinement of neutron diffraction data indicates a significant oxygen vacancy content of y = 0.20(2), with the vacancies distributed at random over the 8a positions. From accurate Ru-O and Bi-O interatomic distances, a bond-valence study suggests an oxidation state lower than 4+ for Ru cations, which is consistent with the presence of oxygen vacancies in the structure, and with the increase of the lattice parameter with respect to stoichiometric . With respect to other ruthenate pyrochlores, the very short bonds between and the special oxygens, in which the electron lone pair of are probably involved, account for the comparatively weak remaining six Bi-O bonds, giving rise to a shift of the O positions and indirectly opening the Ru-O-Ru angles. These angles control the transfer integral between Ru and O 2p orbitals, strongly related to the -block bandwidth and, thus, to the electronic delocalization across the solid. The control of the content in (y depending on the final annealing conditions of the material) may be, therefore, a clean way to tune the bandwidth of this technologically important electronic material.

ChemInform Abstract: Synthesis and X‐Ray Diffraction Study of Potassium, Rubidium, and Cesium Polytungstates with Defect Pyrochlore and Hexagonal Tungsten Bronze Structures.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Crystal Structure and Phase Transition in the Defect Pyrochlore NH4NbWO6

The crystal structure of ammonium tungstoniobate is revised as some features observed in its X-ray diffraction pattern are not consistent with the extinction conditions of space groupFd3m, commonly assigned to this compound. A better agreement between the observed and calculated X-ray powder diffraction patterns was obtained by performing a Rietveld refinement with a structural model using the noncentrosymmetric space groupF43m. The temperature dependence of theFd3m-forbidden 420 Bragg peak intensity indicates a continuous transition at 455 K from the space groupF43mto a more symmetrical, high-temperature phase, with space groupFd3m.

ChemInform Abstract: Complex Oxides of Tungsten, Cesium (Rubidium), and Divalent Metals with a Defect Pyrochlore Structure.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hydrothermal Synthesis and Structure of a Potassium Tantalum Defect Pyrochlore

The synthesis and crystal structure of potassium tantalate (KTa(2)(O,OH)(6).1.8H(2)O) with a defect pyrochlore structure are reported. The compound was crystallized directly from tantalum pentoxide in basic solution under hydrothermal conditions at temperatures as low as 200 degrees C. Ion exchange of this substance in acid yielded an H-type defect pyrochlore. Both highly crystalline K-type and H-type materials absorb argon and have BET surface areas of about 14 m(2)/g. The crystal structures were refined by the Rietveld least-squares method from powder X-ray diffraction data. The final R(wp) and chi(2) were 7.70% and 2.79, respectively. The high thermal stability and control of acidity of these materials, along with the simple and cheap synthesis, may find many applications in sorption, catalysis, and microelectronics.

Conductivity studies on AgSbO 3 channel structure by impedance spectroscopy

The conductivity mechanism of AgSbO 3 with defect pyrochlore structure was examined using complex impedance spectroscopy. The stoichiometric compound is a channel-type Ag +-ion conductor, while thermal treatment of the samples leads to the formation of metallic silver islands by autoreduction transferring the polycrystalline material into a predominantly electronic conductor. Electron transport originates from electronic hopping supported by the metallic islands in the grain boundary phase.