Type II Bi 1- x W x O 1.5 + 1.5 x : a (3 + 3)-dimensional commensurate modulation that stabilizes the fast- ion conducting delta phase of bismuth oxide

The Type II phase in the Bi1 xWxO1.5 + 1.5x system is shown to have a (3 + 3)- dimensional modulated -Bi2O3-related structure, in which the modulation vector " ‘locks in’ to a commensurate value of 1/3. The structure was refined in a 3 3 3 supercell against single-crystal Laue neutron diffraction data. Ab initio calculations were used to test and optimize the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be essentially the same as for the recently refined (3 + 3)-dimensional modulated structure of Type II Bi1 xNbxO1.5 + x (Ling et al., 2013), based on a transition from fluorite-type to pyrochlore-type via the appearance of W4O18 ‘tetrahedra of octahedra’ and chains of corner-sharing WO6 octahedra along h110iF directions. The full range of occupancies on this mixed Bi/W site give a hypothetical solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185), consistent with previous reports and with our own synthetic and analytical results

Cation distributions and anion disorder in Ba3NbMO8.5 (M = Mo, W) materials: Implications for oxide ion conductivity

Competitive oxide ion conductivity has been identified recently in members of the Ba3Nb1–y(Mo1– xWx)1+yO8.5+½y (0 ≤ x ≤ 1, –0.3 ≤ y ≤ 0.2) series, which adopt a disordered rhombohedral “hybrid” structure combining features of the 9R perovskite and palmierite structures. We report the first growth of Ba3NbMoO8.5 and Ba3NbWO8.5 single crystals from molten phases and their characterisation using single-crystal x-ray diffraction data between 120 and 473 K. Structure refinements reveal a previously unreported splitting of the central Nb/M cation site, rationalised by bonding considerations, which imposes limitations on the material stoichiometry and possible arrangements of cations within the face-sharing polyhedral stacks. Analysis of atomic displacement parameters and bond valence energy landscapes (BVELs) gives new insight into the probable low-energy pathways for oxide ion diffusion in the hybrid structure, indicating that they are three-dimensional and involve all crystallographically distinct oxygen sites. Evidence for considerable static disorder of the oxide ions at temperatures below the onset of significant conductivity is also discussed

Exploring the nature of the fergusonite–scheelite phase transition and ionic conductivity enhancement by Mo6+ doping in LaNbO4

A number of metal oxides that crystallise in the scheelite structure type are known to be excellent oxide ion conductors. Here we report the synthesis of a series of materials with general formula LaNb1−xMoxO4+0.5x (x = 0, 0.08, 0.12, 0.16, 0.20) and excellent oxide-ionic conductivity for x ≥ 0.16 (7.0 × 10−3 S cm−1 at 800 °C). Bond valence energy landscape analysis showing possible facile oxide ion migration pathways give important insights into the local influence of defects on oxide-ionic conductivity in these phases. We also use variable-temperature powder X-ray diffraction data to present, for the first time for any scheelite-type material, a symmetry distortion mode refinement-based analysis of the phase transition between the scheelite and fergusonite structure types. This structural phase transition is known to have implications for both oxide-ionic conductive and ferroelastic properties. We demonstrate that one particular distortion mode, namely the Γ2+ displacive mode of the Nb atoms, is the most significant structural distortion leading to the symmetry-breaking phase transition from the tetragonal scheelite to the monoclinic fergusonite form of the material. Our diffraction data and ab initio lattice dynamics calculations provide evidence that the fergusonite–scheelite transition in these materials exhibits characteristics of a first-order transition

Order, Disorder, and Dynamics in Brownmillerite Sr2Fe2O5

The room-temperature structure of brownmillerite-type Sr2Fe2O5 remains controversial, despite numerous published crystallographic studies utilizing X-ray, neutron, and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO4 tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the reassignment of satellite reflections, which harmonize the results of past studies in a unified description of tetrahedral chain ordering in Sr2Fe2O5 at length scales relevant to X-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed

A combined experimental and computational study of oxide ion conduction dynamics in Sr2Fe2O5 brownmillerite

We report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr2Fe2O5, including lat-tice anisotropy, based on neutron scattering studies of a large (partially twinned) single crystal in combination with ab initio molecular dynamics simulations. Single-crystal diffraction reveals supercell peaks due to long-range order-ing among chains of corner-sharing FeO4 tetrahedra, which disappears on heating above 540 °C due to confined local rotations of tetrahedra. Our simulations show that these rotations are essentially isotropic, but are a precondition for the anisotropic motion that moves oxide ions into the tet-rahedral layers from the octahedral layers, which we observe experimentally as a Lorentzian broadening of the quasielas-tic neutron scattering spectrum. This continual but incoher-ent movement of oxide ions in turn creates conduction pathways and activates long-range diffusion at the interface between layers, which appears to be largely isotropic in two dimensions, in contrast with previously proposed mecha-nisms that suggest diffusion occurs preferentially along the c axis

Continuous negative-to-positive tuning of thermal expansion achieved by controlled gas sorption in porous coordination frameworks

Achieving control over the thermomechanical properties of functional materials is desirable, yet remains highly challenging. Here, the authors demonstrate continuous negative-to-positive tuning of thermal expansion in two Prussian blue analogues, by varying the concentration of adsorbed CO2

Hexagonal perovskite related oxide ion conductor Ba3NbMoO8.5: phase transition, temperature evolution of the local structure and properties

Ba3NbMoO8.5 has recently been demonstrated to exhibit competitive oxide ion conductivity and to be stable under reducing conditions, making it an excellent potential electrolyte for solid oxide fuel cells. We report here the first investigation of the local structure in Ba3NbMoO8.5, carried out using variabletemperature neutron total scattering and pair distribution function (PDF) analysis. This work reveals a significant degree of disorder in the material, even at ambient conditions, in both the cation and the anion arrangements and suggests the prevalence of the five-fold Nb/Mo coordination. In addition, high resolution powder X-ray diffraction data indicate that the temperature-dependent structural changes in Ba3NbMoO8.5 are due to a first order phase transition, and reveal a previously unreported effect of thermal history on the room-temperature form of the material. PDF modelling shows that Ba3NbMoO8.5 has an essentially continuous oxygen distribution in the ab plane at 600 C which leads to its high oxideion conductivity