Synthesis of novel cation-ordered compounds with fluorite-related structure prepared by oxidation of Sn-Ta-O pyrochlore

Pyrochlore-type oxide, Sn^II_1.64(Ta_1.88Sn^IV_0.12)O_6.58 was oxidized for exploring novel metastable phases. Two novel Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O_4.11 phases with a fluorite-related structure, which was obtained as a pure phase, and with a rutile-related structure appearing as an impurity for the fluorite-related phase were successfully obtained similar to the previous Sn-Nb-O system. The ordered arrangement of respective cations of Sn and (0.94Ta+0.06Sn) in the precursor Sn^II_1.64(Ta_1.88Sn^IV_0.12)O_6.58 was left in the both oxidized Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O_4.11 phases. In contrast to the previous Sn-Nb-O system, the cation-ordered α-PbO₂-related Sn^IV_0.82(Ta_0.94Sn^IV_0.06)O_4.11 could not be obtained in the present conditions. Such a difference between the Sn-Ta-O and Sn-Nb-O systems was interpreted by larger energy barrier of the transformation from the fluorite-related phase to α-PbO₂-related phase in Sn-Ta-O system than in Sn-Nb-O system.     

Estimation of conductivity parameters in Sr1 − xThxF2 + 2x solid solution with fluorite-type structure

Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr_{1 − x}Th_xF_{2 + 2x} solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F_i)_m ions responsible for long range motions in Sr_{1 − x}Th_xF_{2 + 2x} are interstitial fluoride ions of F type.     

现场测定天然水中微量氟的三点法

提出了在水源现场测定微量氟离子的离子选择性电极三点法，该法受测量条件影响较小，且操作简单，快速、灵敏度高，特别适用于野外现场分析。     

Polymorphism and high-temperature conductivity of Ln2M2O7 (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores

We present a comparative analysis of the order─disorder transitions in Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) pyrochlore-like compounds and solid solutions existing in the Ln₂O₃─MO₂ systems. In the range ~ 600─1200 °C, Ln₂Ti₂O₇ (Ln = Sm─Lu) and Ln₂Zr₂O₇ (Ln = Sm─Gd) undergo ordering transitions, F? → PI → P, which culminate in the formation of an ideal pyrochlore structure, P, existing between 1100 and 1300 °C. Above 1300 °C, Ln₂Ti₂O₇ (Ln = Gd─Lu), Ln₂Zr₂O₇ (Ln = Sm─Gd) and Ln₂Hf₂O₇ (Ln = Eu─Tb) exist as oxygen-ion-conducting phases, PII, disordered in both the oxygen and cation sublattices. Ionic conductivity data for Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) synthesized at 1600-1670 °C indicate that the highest conductivity in these systems is typically offered by nominally stoichiometric (Ln:M = 1:1), disordered Ln₂M₂O₇ (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores containing anti-structure pairs (Ln_M^' + M_Ln^•) and oxygen vacancies (V_O^••) on the 48f (O2) site. The highest conductivity of Yb₂Ti₂O₇, in which the cations have the smallest radii among the lanthanides and Group IVa metals, seems to be due to the increased role of the geometric factor in the Ln₂Ti₂O₇ (Ln = Sm-Lu) pyrochlores with predominantly covalent metal─oxygen bonding M-O (Ti-O). The ion transport parameters in these materials are determined primarily by the relationship between the sizes of the mobile oxygen ions and conduction channels.     

A neutron total scattering study of defect structure in Bi3Nb0.5Y0.5O6.5

The defect structure of the title compound has been analyzed by reverse Monte Carlo (RMC) modeling of neutron total scattering data. The composition exhibits both diffuse scattering and weak superlattice ordering of the cubic fluorite subcell in neutron diffraction patterns. Combined Rietveld analysis of X-ray and neutron data at room temperature reveals oxide ion scattering on three crystallographic sites. Analysis of the RMC model reveals Bi coordination numbers consistent with stereochemical activity of the Bi 6 s² lone pair electrons. Integration of the O-M-O angular distribution function gives an angular ratio consistent with predominantly < 110> vacancy ordering in this system.     

New anion-conducting solid solutions Bi1−xTex(O,F)2+δ (x > 0.5) and glass–ceramic material on their base

The anion-excess fluorite-like solid solutions with general composition Bi_1−xTe_x(O,F)_2+δ (x > 0.5) have been synthesized by a solid state reaction of TeO₂, BiF₃ and Bi₂O₃ at 873 K with following quenching. The homogeneity areas and polymorphism of the I ↔ IV Bi_1−xTe_x(O,F)_2+δ phases were investigated. The crystal structure of the low temperature IV-Bi_1−xTe_x(O,F)_2+δ phase has been solved using electron diffraction and X-ray powder diffraction (a = 11.53051(9) Å, S.G. Ia-3, R_I = 0.046, R_P = 0.041). Glass formation area in the Bi₂O₃–BiF₃–TeO₂ (10% TiO₂) system was investigated. IVBi_1−xTe_x(O,F)_2+δ phase starts to crystallize at short-time (0.5–3 h) annealing of oxyfluoride glasses at temperatures above T_g (600–615 K). The ionic conductivity of the crystalline Bi_1−xTe_x(O,F)_2+δ phase and corresponding glass-ceramics was investigated. Activation energy of conductivity E_a = 0.41(2) eV for the IV-Bi_1−xTe_x(O,F)_2+δ crystalline samples and E_a = 0.73 eV for the glass-ceramic samples were obtained. Investigation of the oxyfluoride samples with a constant cation ratio demonstrates essential influence of excess fluorine anions on the ionic conductivity.     

Thermal variation of structure and electrical conductivity in Bi14WO24

Structure and electrical conductivity of Bi₁₄WO₂₄ as a function of temperature have been examined by X-ray and neutron powder diffraction, a.c. impedance spectroscopy and differential thermal analysis. The room temperature structure was successfully refined using a monoclinic subcell model in space group I2/m. However, additional reflections in the neutron data are consistent with a large supercell of dimensions a = 17.3780(1) Å, b = 17.3891(1) Å, c = 26.1785(2) Å and β = 90.270(1)°, as previously proposed. Transitions to tetragonal and cubic phases are observed at ca. 35 °C and 780 °C, respectively. The structure of the high temperature polymorph is confirmed as a fully disordered δ-Bi₂O₃ type phase. Analysis of the defect structure is consistent with a predominantly tetrahedral environment for tungsten, as seen at low temperatures. The conductivity behaviour is correlated with the appearance of the δ-phase at high temperatures and exhibits a value of 0.97 S cm^− 1 at 800 °C.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Phase and electrical behaviour in the Bi14W1 − xLaxO24 − 3x/2 system

The compositional and thermal dependencies of phase and electrical behaviour of compositions in the system Bi₁₄W_1 − xLa_xO_{24 − 3x/2} (0.00 < x < 1.00) have been studied by X-ray powder diffraction, differential thermal analysis and a.c. impedance spectroscopy. The system exhibits polymorphism and phase separation, which shows both compositional and thermal dependence. Compositions with x = 0.25 and x = 0.50 exhibit a single phase tetragonal structure at room temperature. In contrast, the x = 0.75 composition at room temperature shows a mixture of a cubic phase and a secondary β-Bi₂O₃ related tetragonal phase. A full solid solution is observed at high temperatures, corresponding to the occurrence of a δ-Bi₂O₃ type phase. The appearance of the various phases correlates well with the observed electrical behaviour. The x = 0.75 composition exhibits exceptionally high conductivity at high temperatures (σ₈₀₀ = 1.34 S cm^− 1), but also shows significant phase separation at lower temperatures.