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

Recent developments in oxide ion conductors: focusing on Dion-Jacobson phases

Oxide-ion conductors, also known as "oxygen ion conductors," have garnered significant attention in recent years due to their extensive applications in a variety of electrochemical devices, including oxygen concentrators, solid-oxide fuel cells (SOFCs), and solid oxide electrolysis cells. The key to improving the performance of these devices is the creation of novel oxide-ion conductors. In this feature article, we discuss the recent developments of new structural families of oxide-ion conductors and of the Dion-Jacobson-type layered oxide-ion conductors with a particular emphasis on CsM₂Ti₂NbO_10-δ (M = Bi and lanthanoids; δ represents oxygen-vacancy content) and their solid solutions. CsBi₂Ti₂NbO_10-δ is the first example of an oxide-ion conductor with a Dion-Jacobson-type layered perovskite structure, and the structural characteristics of these materials are extracted here. We have proposed an original concept that the large sized Cs⁺ cations and M³⁺ displacements yield the large bottlenecks for oxide-ion migration, which would facilitate the discovery of novel oxide-ion conductors. This article presents evidence that Dion-Jacobson-type layered perovskites are superior oxide-ion conductors. We also demonstrate how the information gleaned from these studies can be applied to the design of novel oxide-ion conductors.

Simultaneous Reduction of Proton Conductivity and Enhancement of Oxide-Ion Conductivity by Aliovalent Doping in Ba7Nb4MoO20

Hexagonal perovskite-related oxides have garnered a great deal of research interest because of their high oxide-ion conductivity at intermediate temperatures, with Ba₇Nb₄MoO₂₀ being a notable example. However, concomitant proton conduction in Ba₇Nb₄MoO₂₀ may cause a decrease in power efficiency when used as the electrolyte in conventional solid oxide fuel cells. Here, through investigations of the transport and structural properties of Ba₇Nb_4-xW_xMoO_20+x/2 (x = 0-0.25), we show that the aliovalent substitution of Nb⁵⁺ by W⁶⁺ not only increases the oxide-ion conductivity but also dramatically lowers proton conductivity. The highest conductivity is achieved for x = 0.15 composition, with 2.2 × 10^-2 S cm^-1 at 600 °C, 2.2 times higher than that of pristine Ba₇Nb₄MoO₂₀. The proton transport number of Ba₇Nb_3.85W_0.15MoO_20.075 is smaller compared with Ba₇Nb₄MoO₂₀, Ba₇Nb_3.9Mo_1.1O_20.05, and Ba₇Ta_3.7Mo_1.3O_20.15. The structure analyses of neutron diffraction data of Ba₇Nb_3.85W_0.15MoO_20.075 at 25 and 800 °C reveal that the aliovalent W⁶⁺ doping introduces interstitial oxide ions in the intrinsically oxygen-deficient c' layers, thereby simultaneously increasing the carrier concentration for oxide-ion conduction and decreasing oxygen vacancies responsible for dissociative absorption of water. Neutron scattering length density distribution was examined using the maximum-entropy method and neutron diffraction data at 800 °C, which indicates the interstitialcy oxide-ion diffusion in the c' layers of Ba₇Nb_3.85W_0.15MoO_20.075. Ba₇Nb_3.85W_0.15MoO_20.075 exhibits extremely high chemical and electrical stability in the wide oxygen partial pressure P(O₂) region [ex. 10^-23 ≤ P(O₂) ≤ 1 atm at 903 °C]. The present results offer a strategy for developing pure oxide-ion conducting hexagonal perovskite-related oxides for possible industrial applications.

Synthesis conditions impact on Sr11Mo4O23 electroceramic: crystal structure, stability and transport properties

Crystal structure and properties of Sr₁₁Mo₄O₂₃ treated at 1100 and 1400 °C were studied via synchrotron X-ray powder diffraction and thermogravimetric analysis, coupled with mass spectrometry. Synchrotron studies reveal the crystallographic effect of the annealing temperature, showing that the lowest-temperature phase must be defined in a triclinic symmetry, in contrast to the cubic one obtained at 1400 °C. The mass spectrometry allowed the identification of the released compounds during the thermogravimetric analysis, thus unveiling the physicochemical behavior of the sample during the heating process. Furthermore, an aging analysis was made, confirming the superior stability of this sample when it is treated at 1400 °C. Finally, an optimized sintering procedure allowed us to obtain a superior density and hence the highest conductivity measured so far for this system.

High oxide-ion conductivity through the interstitial oxygen site in Ba7Nb4MoO20-based hexagonal perovskite related oxides

Oxide-ion conductors are important in various applications such as solid-oxide fuel cells. Although zirconia-based materials are widely utilized, there remains a strong motivation to discover electrolyte materials with higher conductivity that lowers the working temperature of fuel cells, reducing cost. Oxide-ion conductors with hexagonal perovskite related structures are rare. Herein, we report oxide-ion conductors based on a hexagonal perovskite-related oxide Ba₇Nb₄MoO₂₀. Ba₇Nb_3.9Mo_1.1O_20.05 shows a wide stability range and predominantly oxide-ion conduction in an oxygen partial pressure range from 2 × 10^-26 to 1 atm at 600 °C. Surprisingly, bulk conductivity of Ba₇Nb_3.9Mo_1.1O_20.05, 5.8 × 10^-4 S cm^-1, is remarkably high at 310 °C, and higher than Bi₂O₃- and zirconia-based materials. The high conductivity of Ba₇Nb_3.9Mo_1.1O_20.05 is attributable to the interstitial-O5 oxygen site, providing two-dimensional oxide-ion O1-O5 interstitialcy diffusion through lattice-O1 and interstitial-O5 sites in the oxygen-deficient layer, and low activation energy for oxide-ion conductivity. Present findings demonstrate the ability of hexagonal perovskite related oxides as superior oxide-ion conductors.

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.