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Battery safety is the number one priority for consumers and manufacturers, particularly for large-scale applications like electric vehicles and distributed energy storage systems, where the consequences of thermal runaway events can be devastating. Here we propose a novel approach to prevent battery thermal runaway by using La and Nb co-doped BaTiO3. The material is incorporated into a battery system as a thin film, having no effects on the room temperature operation, but rapidly switching off the battery current at high temperatures due to the positive temperature coefficient of resistivity (PTCR) exhibited by doped BaTiO3. La and Nb as co-dopants of BaTiO3 are found to be critical to ensure good room temperature conductivity combined with a significant PTCR effect. This work demonstrates the use of a purely inorganic PTCR material for thermal runaway protection for the first time. The high mechanical and chemical stability of the BaTiO3-based material proposed here makes it an advantageous competitor to current polymer-based protective switches.

The hexagonal perovskite derivatives Ba3NbMoO8.5, Ba3NbWO8.5, and Ba3VWO8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba3-xVMoO8.5-x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1Ox polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba3M'M″O8.5 family, Ba3-xVMoO8.5-x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba3-xVMoO8.5-x is unstable at lower temperatures, as illustrated by the formation of BaCO3 and BaMoO4 by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba3-xVMoO8.5-x.

Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba

Ba3MoNbO8.5 is an oxide ion conductor with an unconventional hybrid crystal structure that is intermediate between the 9R-perovskite (A3B3O9) and the palmierite (A3B2O8). The crystal structure is highly disordered with vacancies distributed across two cation (M(1) and M(2)) and oxygen sites (O(2) and O(3)), with Mo and Nb in variable coordinate environments. M(1)–O(1)–O(2) and M(2)–O(1) sites are associated with the formation of (Mo,Nb)O6 octahedra, whilst tetrahedral units are composed of M(1)–O(1)–O(3) atoms. Upon increasing the temperature, the structure undergoes a change in occupancy in favour of the O(3) site, which results in a change in metal co-ordination as the tetrahedral to octahedral ratio increases. We demonstrate that the structure can also be tuned using externally applied pressure. Variable pressure studies ≤4.8 GPa indicate that densification of the unit cell induces the reverse effect, as the occupancy of the O(2) site increases and the palmierite contribution is suppressed. Our results strongly suggest that by 5.2 GPa the O(3) position will be completely empty as the 9R unit cell stabilises with a network of octahedral MO6 units. Pressure induces a flattening of M(1)O4 tetrahedra in the palmierite layers, as M(1)O6 octahedra become more regular in geometry. Bond valence site energy calculations show that pressure increases the height of all energy barriers to migration along the three-dimensional diffusion pathways, increasing the energy of the dominant pathway from 0.35 to 0.95 eV. The relaxation energy, E2, disappears above 2.8 GPa, when the average polyhedral distortion (σ(R)) falls below 0.07 A, indicating the existence of a critical minimum. The bulk modulus of Ba3MoNbO8.5 is exceptionally low (50(2) GPa) for a layered oxide material and is closer to that of the halide perovskites. These results demonstrate a high degree of flexibility, in terms of the softness of the lattice and variable metal coordination, emphasising the potential for these materials in multi sensory and thin film applications.

Solid oxide proton conductors are crucially emerging as key materials for enabling hydrogen-based energy conversion, storage, and electrochemical technologies. Oxides crystallising in the ideal ABO3 perovskite structure, such as barium cerates and zirconates, are widely investigated thanks to their excellent proton conducting properties. Nevertheless, alternative structure-type solid oxide systems (hexagonal perovskite derivatives, brownmillerite, scheelite, etc.) can efficiently incorporate and enable the transport of protonic defects, with recent reports of materials exhibiting high ionic conductivity comparable to the conventional perovskite conductors. This perspective provides an overview of these alternative and less established proton conducting materials, with particular attention to the relationship between the structural and ionic conduction features and the mechanistic aspects. The goals are to highlight the differences between these materials and the traditional perovskites and to point out new potential crystal routes for the discovery of innovative solid oxide proton conductors.

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy related technologies. Here we report on the discovery of significant proton and oxide ion conductivity in palmierite ox-ides A3V2O8 (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO4 units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (tH = 0.6 – 0.8) and high protonic mobility. In particular, the proton conductivity of Sr3V2O8 is 1.0 x 10-4 S cm-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy driven and is facilitated by rotational motion of the VO4 units, which can stabilize oxygen defects via formation of V2O7 dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

Significant oxide ion conductivity has recently been reported in the cation-deficient hexagonal perovskite derivative Ba3NbMoO8.5. This system exhibits considerable anion and cation disorder. Oxyge...

Significant oxide ionic conductivity has recently been reported in cation-deficient hexagonal perovskite Ba3M′M′′O8.5 derivatives (M′ = Nb; M′′ = Mo, W), with disordered hybrid 9R-palmierite average structures. Here, we present a study of the crystal structure and electrical properties of the related compound Ba3VWO8.5. Electrical characterization demonstrates that Ba3VWO8.5 is also an oxide ion conductor with a bulk conductivity of 2.0 × 10−3 S cm−1 in air at 900 °C, thus revealing that it is possible to obtain oxide ion conducting Ba3M′M′′O8.5 materials with a variety of different M′M′′ combinations. Whilst Ba3NbMoO8.5 and Ba3NbWO8.5 present a random distribution of cationic vacancies, X-ray and neutron diffraction experiments demonstrate that the cationic vacancies are ordered on the M2 sites in Ba3VWO8.5, resulting in a structure where M1Ox palmierite-like layers are separated by empty octahedral cavities. Bond-valence site energy (BVSE) analysis on the different phases reveals that ordering of the cationic vacancies hinders long-range oxygen diffusivity parallel to the c-axis in Ba3VWO8.5 explaining the reduced ionic conductivity of this compound. These results suggest that, together with the dominant 2-dimensional conduction pathway along the palmierite-like layers, additional diffusion routes parallel to the c-axis provide a relevant contribution to the conductivity of these Ba3M′M′′O8.5 systems by creation of a complex 3-dimensional ionic percolation network, the topology of which depends on the particular arrangement of cation and anion vacancies.

Ba

The hexagonal perovskite derivatives Ba

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm−1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies. Fast oxide ion and proton conductors at intermediate temperature are required to improve the performance of ceramic fuel cells. An undoped hexagonal perovskite Ba7Nb4MoO20 electrolyte with high proton and oxide ion conductivity (4.0 mS cm−1) at 510 °C is now reported.

The structural and electrical properties of the hexagonal perovskite derivative Ba3WNbO8.5 have been investigated. Ba3WNbO8.5 crystallises in a hybrid of the 9R hexagonal perovskite and palmierite structure as recently reported for the novel oxide ion conductor Ba3MoNbO8.5. Ba3WNbO8.5 is also an oxide ion conductor and appears to exhibit oxide ionic conduction with negligible electronic conductivity over a wider pO2 range than Ba3MoNbO8.5. A neutron diffraction study has revealed that at 20 °C the average structure of Ba3WNbO8.5 contains just 13% of W(1)/Nb(1)O4 tetrahedra within the average structure of Ba3WNbO8.5 in comparison to 50% of Mo(1)/Nb(1)O4 tetrahedra in Ba3MoNbO8.5. The presence of (M/Nb)O4 tetrahedra with non-bridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MNbO8.5 system (M = W, Mo). The strong reduction in the ratio of (M/Nb)O4 tetrahedra to (M/Nb)O6 octahedra upon replacement of W6+ for Mo6+ results in a reduction in the ionic conductivity by an order of magnitude at 450 °C. The bulk conductivities converge upon heating so that at 600 °C the bulk conductivity of Ba3WNbO8.5, σb = 0.0017 S cm−1, is comparable to that of Ba3MoNbO8.5 (σb = 0.0022 S cm−1). The results demonstrate that other members of the Ba3MM′O8.5 family can support oxide ion conductivity and further studies of hexagonal perovskite derivatives are warranted.

The electrical and structural properties of the series Ba3Mo1–xNb1+xO8.5–x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba3Mo1–xNb1+xO8.5–x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O4 and (Mo/Nb)O6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10–6 S cm–1 for x = 0.0 to 1.1 × 10–7 S cm–1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba3MoNbO8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba3MoNbO8.5 system.

Various structural families have been reported to support oxide ion conductivity; among these, perovskite conductors have received particular attention. The perovskite structure is generally composed of a framework of corner-sharing octahedral units. When the octahedral units share their faces, hexagonal perovskites are formed. Mixed combinations of corner-sharing and face-sharing octahedral units can give rise to a variety of hexagonal perovskite derivatives. However, the ionic conducting properties of these materials have not been well explored. In this feature article, we review the conducting properties of the most significant hexagonal perovskite derivatives, with special focus on Ba3MM′O8.5. Ba3MM′O8.5 is the first hexagonal perovskite derivative to exhibit substantial oxide ion conductivity, and here we outline the structural features that are key for the oxide ion conduction within this system. The results demonstrate that further investigation of hexagonal perovskite derivatives could open up new directions in the design of oxide ion conductors.

Nanosized α-zirconium phosphate, α-ZrP, undergoes a phase transition at 120 °C, which is not observed with microcrystalline α-ZrP in the same conditions, and which leads to a new 3D phase. The new compound, with formula Zr(HPO4)2 (τ'-ZrP), consists of cubelike nanoparticles and has a tetragonal unit cell (space group P43212, a = 7.955 Å, c = 10.744 Å). The structure of τ'-ZrP is in close relationship with that of the already known τ-ZrP. Both structures are made of packed chains of eight-membered rings, composed of Zr atoms connected to bridging HPO4 groups. The main difference between the two structures concerns the different orientation of the uncoordinated P-OH groups, pointing into the channels. The in situ XRPD analysis on nanosized α-ZrP, performed at 120 °C as a function of time, provided information about the kinetics of the formation of τ'-ZrP, showing that the α-ZrP phase is directly transformed into τ'-ZrP. Moreover, τ'-ZrP is converted into α-ZrP at room temperature in the presence of water vapor. It was proved that the free phosphoric acid, which is originally present in small amounts in nanosized α-ZrP and τ'-ZrP, is necessary for the interconversion between the two phases. As a matter of fact, the removal of phosphoric acid, by washing α-ZrP and τ'-ZrP with anhydrous ethanol, inhibits the above conversion.

The electrical and structural properties of the series Ba

This research was supported by the Northern Research Partnership and the University of Aberdeen. We also acknowledge STFC-GB for provision of beamtime at the ILL. A variable temperature neutron diffraction study of the novel oxide ion conductor Ba3MoNbO8.5 has been performed between 25 and 600 °C. Nonmonotonic behavior of the cell parameters, bond lengths, and angles are observed indicating a structural rearrangement above 300 °C. The oxygen/vacancy distribution changes as the temperature increases so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra increases upon heating above 300 °C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)O4 tetrahedra within the average structure of Ba3MoNbO8.5 is observed. The increase in the number of (Mo/Nb)O4 tetrahedra upon heating from 300-600 °C most likely offers more low energy transition paths for transport of the O2- ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, increasing the ionic conductivity. The second order Jahn-Teller effect most likely further enhances the distortion of the MO4/MO6 polyhedra as distortions created by both electronic and structural effects are mutually supportive. Postprint

This research was supported by the Northern Research Partnership and the University of Aberdeen. Oxide ion conductors are important materials with a range of technological applications and are currently used as electrolytes for solid oxide fuel cells and solid oxide electrolyzer cells. Here we report the crystal structure and electrical properties of the hexagonal perovskite derivative Ba3MoNbO8.5. Ba3MoNbO8.5 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures. This is a new and so far unique crystal structure that contains a disordered distribution of (Mo/Nb)O6 octahedra and (Mo/Nb)O4 tetrahedra. Ba3MoNbO8.5 shows a wide stability range and exhibits predominantly oxide ion conduction over a pO2 range from 10-20 to 1 atm with a bulk conductivity of 2.2 × 10-3 S cm-1 at 600 °C. The high level of conductivity in a new structure family suggests that further study of hexagonal perovskite derivatives containing mixed tetrahedral and octahedral geometry could open up new horizons in the design of oxygen conducting electrolytes. Postprint

Oxide ion conductors are important materials with a range of technological applications and are currently used as electrolytes for solid oxide fuel cells and solid oxide electrolyzer cells. Here we report the crystal structure and electrical properties of the hexagonal perovskite derivative Ba

Microcrystalline zirconium phosphate was exfoliated by treatment with aqueous solutions of α,ω-alkylaminoalcohols and employed for the fabrication of potato starch composite membranes. Glycerol-based and glycerol-free composite membranes, containing 5 wt% of filler, were prepared from gelatinized starch and characterized for their physico-chemical properties. Despite of a partial filler reaggregation, as revealed by XRD and SEM analysis, all the composites exhibited a significant increase in the Young's modulus with respect to the glycerol-starch membrane, up to 80% and 190% for the glycerol-based and the glycerol-free composites, respectively. For both kinds of membranes the filler delays to a large extent the starch decomposition above about 300 °C. A significant reduction in the water uptake of the composites was also observed with respect to the neat glycerol-based membrane, up to about 70% for the glycerol-free composites.