Novel perovskite-related barium tungstate Ba11W4O23

Multi-scale structural analysis of the A-site and oxygen deficient perovskite Sr11Mo4O23

The long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr₁₁Mo₄O₂₃ have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd3[combining macron]m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr₁₁Mo₄O₂₃ appears to be isostructural with Ba₁₁W₄O₂₃, except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O-O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways.

Structural origin of the enhanced ionic conductivity upon Nb doping in Sr11Mo4O23 defective double perovskite

We report a substantial enhancement of the oxide-ion conductivity in Sr₁₁Mo₄O₂₃ achieved by Nb doping the Mo sites. This series responds to the formula: Sr₁₁Mo_4-xNb_xO_23-δ (with x = 0.0, 0.5 and 1.0). The original structure can be related to the conventional double perovskite; however, it presents a broken corner sharing connectivity of the octahedral framework, hence leading to a complex and highly defective network. The samples were prepared via citrate precursor method, followed by thermal treatments at 1300 °C for 12 hours in air. The crystal structures were refined from X-ray and neutron powder diffraction (NPD) data. A phase transition from tetragonal to cubic symmetry is identified in a temperature-dependent NPD study, driven by an oxygen delocalization effect. The ionic conductivity measured by impedance spectroscopy is enhanced upon Nb-doping; the x = 1 doped phase exhibits a threefold increase compared to the pristine Sr₁₁Mo₄O₂₃ oxide, with conductivity values of 7.6 × 10^-3 and 2.7 × 10^-2 S·cm^-1 at 650 and 800 °C, which are even greater than for YSZ in the 650-800 °C temperature range, and close to those reported for other state-of-the art solid-oxide electrolytes.

Ionic conductivity enhancement in Ti-doped Sr11Mo4O23 defective double perovskites

A substantially higher ionic motion can be achieved by partially replacing Mo(vi) by Ti(iv) cations in the novel Sr₁₁Mo_4−xTi_xO_23−δ (with x = 0.0, 0.5 and 1.0) electrolyte oxides, successfully enhancing the oxygen vacancy level.

High-temperature dynamic octahedral tilting in the ionic conductor Sr11Mo4O23

This work presents the crystal structure evolution of a novel ionic conductor Sr11Mo4O23at high temperature. The formula of this phase can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskitesA2B′B′′O6. The crystal network contains oxygen-anion and strontium-cation vacancies. The structure is complex; Sr, Mo and O atoms are distributed in four, two and six distinct Wyckoff sites, respectively. It was refined from neutron powder diffraction data collected at 473, 673, 873 and 1073 K. The thermal evolution of crystallographic parameters supports the known reversible process of removal/uptake of O-atom content in the 673–873 K temperature range. Above 873 K, from difference Fourier maps, it was found that the structure exhibits an oxygen delocalization around one of the Mo sites. This delocalization was understood as a dynamical octahedral tilt of the MoO6octahedron, yielding an increase in the ionic conductivity at high temperature.

The strongly defective double perovskite Sr11Mo4O23: crystal structure in relation to ionic conductivity

The crystal structure and ionic conductivity properties of a novel microcrystalline Sr11Mo4O23ceramic material are presented. This material has been prepared by thermal treatment up to 1473 K, in air, of previously decomposed citrate precursors. The complex crystal structure was refined from combined X-ray powder diffraction and neutron powder diffraction data. The formula of this phase can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskitesA2B′B′′O6. At room temperature, the crystal structure is tetragonal in space groupI41/a, witha= 11.6107 (6) Å,c= 16.422 (1) Å andV = 2213.8 (2) Å3. The crystal network contains O anion and Sr cation vacancies. The structure is complex, with Sr, Mo and O atoms distributed over four, two and six distinct Wyckoff sites, respectively. Only one of the Sr sites (SrO6) corresponds to the octahedral network; one of the two MoO6types of octahedra is axially distorted. The three other Sr positions occupy theAsite with higher coordination. There is an occupational deficit of O atoms of 22 (4)%. This defective framework material presents an interesting ionic mobility, enhanced above 773 K owing to a further reduction in the oxygen content.