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Abstract: 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 0.97Scm−1 at 800°C. [Copyright &y& Elsevier]

Abstract: The structure of Bi3TaO7 has been examined by X-ray and neutron powder diffraction. The structure is that of an ordered defect fluorite and has been refined using a cubic δ-Bi2O3 type subcell model. A defect structure model is proposed with Ta5+ in predominantly a distorted octahedral geometry. These tantalate octahedra are thought to form corner sharing chains in a similar way to that proposed in the isostructural phase Bi3NbO7. The thermal variation of conductivity and lattice parameter appear to be correlated with subtle changes in the oxide ion distribution. [Copyright &y& Elsevier]

Abstract: A detailed investigation using X-ray and neutron powder diffraction as well as ac impedance spectroscopy in the substituted bismuth oxide system Bi7Nb2−2x Y2x O15.5−2x is presented. Combined refinement of variable temperature X-ray and neutron powder diffraction data reveal both compositional and temperature dependencies of the oxide ion distribution in this system. Oxide ions are found to be distributed over three crystallographically distinct sites, with one site exclusively associated with the dopant cations Nb5+ and Y3+ leading to a distorted octahedral geometry for these ions. Changes in the oxide ion distribution are correlated with a non-linear thermal expansion as well as non-Arrhenius behaviour of total conductivity at high temperatures. [Copyright &y& Elsevier]

Abstract: Tungsten doping into Bi7Nb2O15.5 was investigated using a combination of X-ray and neutron diffraction and ac impedance spectroscopy. Only limited solid solution formation is observed in the system Bi7Nb2−2x W2x O15.5+ x , with a defect fluorite type phase observed up to around x =0.2. The defect structure of this phase shows, that Nb5+ and W6+ ions are likely to adopt a distorted octahedral coordination geometry, with Bi3+ ions in predominantly four-pyramidal geometry. Non-linearity in the Arrhenius plot of conductivity as well as the thermal variation of the cubic lattice parameter are interpreted in terms of a subtle redistribution of oxide ions in the structure at elevated temperatures. [Copyright &y& Elsevier]

Abstract: Ageing effects in the defect fluorite structure of Bi3Nb1−x Y x O7−x have been studied using powder neutron diffraction. Two compositions at x =0.4 and x =0.6 were studied after annealing for prolonged times (400–500h) at 410 and 550°C. The results show that significant changes in oxide ion distribution occur at 550°C, but that at 410°C only the lower x-value composition exhibited such a change. A comparison of lattice parameter variation with temperature for annealed and unannealed samples suggests that at higher temperatures the oxide ion distribution is independent of thermal history. [Copyright &y& Elsevier]

Abstract: An investigation of electrical conductivity and defect structure in the fluorite related system Bi3+ x Nb0.8W0.2O7.1+3x/2 (0.0≤ x ≤2.0) using a.c. impedance spectroscopy and powder neutron diffraction is presented. All compositions exhibit incommensurate ordering of the cubic fluorite sublattice, with only the x =2.0 composition showing evidence for an order–disorder transition above 650°C. Non-linear behaviour in the thermal expansion of the cubic lattice parameter and the Arrhenius plot of total conductivity are discussed in terms of the defect structure. [ABSTRACT FROM AUTHOR]

Abstract: Double substitution of bismuth in bismuth oxide is investigated in the system Bi3Nb1− x Er x O7− x using X-ray and neutron diffraction and a.c. impedance spectroscopy. A full solid solution is observed with the basic fluorite structure maintained throughout. Long-range superlattice ordering observed at low x-value compositions is replaced by short-range ordering as the erbium content increases. Nb5+ and Er3+ dopant cations are believed to adopt a distorted octahedral coordination geometry, with evidence for clustering of these ions. Significant compositional changes are observed in the oxide ion distribution, but there is little significant thermal change in the average oxide ion distribution to account for observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plot of total conductivity. These phenomena may well be associated with more subtle changes in local ordering of vacancies. [Copyright &y& Elsevier]

Abstract: Defect structure and conductivity behaviour are discussed in the solid solution Bi3Nb1−x Y x O7−x (0.0≤ x ≤1.0). Investigations were carried out using a combination of ac impedance spectroscopy and powder X-ray and neutron diffraction. Low temperature conductivity and activation energy both increase as a function of x. The former is attributed to an increase in oxide ion vacancy concentration, whereas the latter is due a redistribution of oxide ion vacancies as determined by neutron diffraction measurements. The defect structures at room temperature and 800°C are presented. Curvature in Arrhenius plots of conductivity throughout the composition range is associated with a temperature dependent redistribution of oxide ions. [Copyright &y& Elsevier]

Abstract A study of structure and oxide-ion conductivity in the Bi 4 Nb 1 − x Y x O 8.5 − x solid solution using X-ray and neutron powder diffraction and a.c. impedance spectroscopy is presented. Slow cooled samples are typically biphasic, exhibiting a mixture of cubic/pseudo-cubic and orthorhombic or tetragonal fluorite based phases. Single phase cubic and pseudo-cubic materials can be isolated by quenching from high temperatures. The defect structure in these quenched phases has been determined by neutron diffraction and shows an oxide-ion distribution that is compositionally dependent. This distribution is correlated with the compositional variation of the low temperature activation energy for total conductivity, whereas the activation energy for high temperature conductivity decreases with increasing value of x , reflecting the reduction in dopant-vacancy interaction with increasing yttrium content, as well as a change from a tetragonally ordered type III phase for low x -value compositions to a fully disordered δ-Bi 2 O 3 type phase at x = 0.4. Ordering phenomena are proposed to account for the appearance of a linear intermediate temperature region with high activation energy in Arrhenius plots of conductivity at x ≥ 0.4. Conductivities in the order of 10−1 S cm−1 are achieved at 700 °C in these materials. Graphical abstract Unlabelled Image Highlights • A full solid solution range is obtained in the system Bi 4 Nb 1 − x Y x O 8.5 − x. • The oxide-ion distribution is compositionally dependent. • Bi 6 s 2 is stereochemically active with Bi coordination numbers between 4.35 and 4.85. • Conductivity increases with increasing vacancy concentration. • Conductivity in the order of 10−1 S cm−1 at 700 °C. [ABSTRACT FROM AUTHOR]

Solid solution formation in the system Bi 3 Y 1 − x W x O 6 + 3 x /2 has been studied using a combination of X-ray and neutron powder diffraction and a.c. impedance spectroscopy. Compositions in the solid solution adopt the δ-Bi 2 O 3 type structure, with single phases evident from x = 0.00 to x = 0.20. Evidence for dopant clustering is presented and discussed. Models for the defect structure derived from diffraction studies are presented. Tungsten is proposed to adopt a tetrahedral coordination geometry, with a distorted octahedral geometry adopted by yttrium. Calculated coordination numbers for bismuth of around five are consistent with stereochemical activity of the Bi 6 s 2 lone pairs of electrons. Despite a significant lowering of the nominal vacancy concentration with respect to δ-Bi 3 YO 6 , as well as enhanced vacancy trapping by W 6+ , tungsten doping is found to have very little influence on the total conductivity of δ-Bi 3 YO 6. This is attributed to the compensating effect of enhanced oxide ion mobility caused by lattice expansion. [ABSTRACT FROM AUTHOR]

Abstract: A study of electrical and structural characteristics of compositions in the Bi3Ta1− x Nb x O7 system, using X-ray and neutron powder diffraction and AC impedance spectroscopy, is presented. The electrical conductivity increases with increasing niobium content. A full solid solution is observed which adopts an incommensurately ordered pseudo-cubic fluorite structure (type II). Analysis of the defect structure of the x =0.50 composition shows chains of niobate/tantalate octahedra as a likely structural motif. A small degree of non-linearity in the thermal expansion of the cubic subcell lattice parameter is observed. [Copyright &y& Elsevier]

Abstract: Structure and electrical behaviour are reported for the system Bi1-x Ta x O1.5+ x (0.167≤ x ≤0.250). In the compositional range 0.200< x ≤0.250 an incommensurately modulated pseudo-cubic phase (type II) is observed, with the appearance of a larger pseudo-cubic phase in the region 0.167≤ x ≤0.200. Structural analysis of the type II phases by neutron diffraction reveals subtle changes in the oxide ion distribution with temperature, associated with changes in the incommensurate modulation parameter. Analysis of the defect structure of the type II phase reveals chains of tantalate octahedra as a likely structural motif. It is proposed that these chains facilitate an electronic contribution to total conductivity at low temperatures through electron hopping along the chains. Changes in oxide ion vacancy ordering may explain the observed non-linear behaviour in the thermal expansion of lattice parameter and Arrhenius plots of total conductivity. [Copyright &y& Elsevier]

Abstract: 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 s 2 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. [Copyright &y& Elsevier]

A high entropy dopant approach has been used to prepare a new BIMEVOX ceramic system, Bi 2 V 1- x (Mg 0.25 Cu 0.25 Ni 0.25 Zn 0.25) x O 5.5-3 x /2. Structures were investigated using a combination of X-ray and neutron powder diffraction, with electrical characterisation by A.C. impedance spectroscopy. A γ-type phase is observed at room temperature over the compositional range 0.10 ≤ x ≤ 0.30, the upper limit of which is beyond that seen for all the single substituted systems based on these substituents, apart from BIMGVOX. No stabilisation of the fully disordered γ-phase is seen at room temperature over this compositional range, with only the incommensurately ordered γ'-phase evident below around 450 °C. Changes in defect structure are used to explain an apparent transition in the compositional variation of lattice parameters. The HE dopant approach has no detrimental effect on ionic conductivity, with values comparable to those of the single substituted systems based on the component oxides. Unlabelled Image • A quadruple substituted BIMEVOX has been created using a High Entropy Dopant approach. • HE approach gives high upper limit of γ-phase solid solution. • Conductivity not degraded by HE approach. • Defect structure of vanadate layer consistent with mixture of MO polyhedra. [ABSTRACT FROM AUTHOR]