Ronald I. Smith

Various hexagonal perovskite derivatives have recently been demonstrated to exhibit high oxide ion conductivity, with their conductivity properties repeatedly being linked to palmierite-like (P-L) structural layers. Here, we report an investigation into the crystal structure and electrical properties of the hexagonal perovskite derivative Ba6Nb2Na2V2O17. Ba6Nb2Na2V2O17 exhibits oxide ion conductivity, with a total conductivity of 1.8 × 10–6 S cm–1 at 650 °C under an atmosphere of flowing dry N2. Unlike other oxide ion conducting hexagonal perovskite derivatives, Ba6Nb2Na2V2O17 is shown not to feature interstitial oxygen, mixed coordination polyhedra, or isolated polyhedra. Regardless, oxide ion conductivity is still enabled by the P-L structural motif. These results hence further emphasize the utility and versatility of P-L layers in the context of ionic conductivity. Our atomistic modeling reveals a distinct vacancy-mediated oxide-ion conduction pathway, predominantly involving the O1 and O2 sites of the P-L layer.

The design and development of new materials is crucial for advanced next-generation technologies, necessitating the modification of materials with tuneable atomic and electronic frameworks. This is often achieved by addition or replacement of metals and cations. Substitution of anions is also possible; here, we present the YBi ₂ O _3.5+δ Se _1-x Cl _x family which represent a new class of layered mixed anionic semiconductor materials with complete substitution of Cl by Se charge balanced by the introduction of oxide vacancies. The introduction of Se anions into the visible band gap material YBi ₂ O ₄ Cl shifts the band gap from 2.4 to 1.2 eV. This coupled Se/Cl substitution with O vacancy generation enables defect-engineered, tuneable new layered materials with tailored electronic properties in the visible band gap region, opening new pathways for development of semiconductors, thermoelectrics, and mixed oxide ion conductors

Advanced characterisation of mechanochemically synthesised LiGaX ₄ halides uncovers their structures, transport properties and key differences between short- and long-range conduction.

A new inverse spinel solid solution Li _{2+ x} Ni _{2−2 x} Cr _x V ₂ O ₈ shows three-dimensional Li ⁺ conduction via 16c–8a–16c pathways, and show potential for next-generation negative electrode applications with stable cycling performance.

Promising ionic conductivity has previously been reported in the palmierite oxide Sr₃V₂O₈. Oxide-ion diffusion within this system occurs via the "cog-wheel" mechanism where rotation of VO₄ and oxygen deficient VO₃ units result in the formation of V₂O₇ dimers which continuously break and reform. During this process an oxide ion from a VO₄ tetrahedra can move to the vacant site of the VO₃ group, facilitating the movement of oxide-ions throughout the structure. Here, we report the electrical and structural properties of the series Sr_3-3x La_2x V₂O₈ (x = 0.00-0.25). Combined neutron diffraction and atomistic simulations reveal that substituting Sr²⁺ with La³⁺ results in ordered cation vacancies, an elongation of the apical V-O1 bond and an increase in the distortion of the VO₄ tetrahedra, which together enhances the cog-wheel-like rotational dynamics of the VO₄ tetrahedra that mediate oxide ion transport. Molecular dynamics simulations further indicate that La-substitution facilitates the formation of a continuous diffusion network, leading to improved oxide ion conductivity so that Sr_2.55La_0.3V₂O₈ (x = 0.15) exhibits the highest bulk conductivity of 7.64 × 10^-4 S cm ^-1 at 700 °C, an order of magnitude higher than Sr₃V₂O₈. The results demonstrate that palmierites are highly flexible to doping strategies for improving the oxide ion conductivity.

Investigation of the earliest stages of hydrothermal reactions between KRuO₄ and either Sr(NO₃)₂ or SrO₂ allows the isolation of phase-pure SrRuO₃(OH)₂, a hitherto uncharacterized Ru(VI) oxyhydroxide, previously mentioned in the literature as SrRuO₄·H₂O. Further exploration of synthesis conditions with excess of strontium salt yields a second Ru(VI) phase, Sr₃Ru₂O₈(OH)₂ at 200 °C. Both structures were solved by 3D-electron diffraction and refined using powder neutron diffraction (SrRuO₃(OH)₂: monoclinic P2₁/n, a = 9.9903(3) Å, b = 7.7023(2) Å, c = 17.3677(6) Å, β = 89.353(2)°; Sr₃Ru₂O₈(OH)₂: tetragonal P4̅, a = 13.2206(5) Å c = 5.4852(2) Å). The two Ru(VI) materials each contain isolated trigonal bipyramidal ruthenium centers with axially positioned hydroxides with longer Ru-OH bond distances than the equatorial Ru-O distances. Infrared spectroscopy corroborates the presence of hydroxide, with O-H stretching bands that are red-shifted upon deuteration. X-ray absorption near-edge spectroscopy at the Ru K-edge confirms the +6 oxidation state of Ru, and magnetization measurements show two unpaired electrons per Ru associated with the 4d² electronic configuration in both materials. The crystal structures, particularly the local environment of Ru, are compared with other reported Ru(VI) compounds, and derived bond valence sum parameters give consistent results for ruthenate(VI) oxides and oxyhydroxides.

La_xSr_2−xCo_0.5Ir_0.5O_4−yH_y oxyhydride phases have hydride ions located exclusively on their equatorial anion sites.

Lithium silicides exhibit much higher thermal expansion than silicon, similar to lithium, consistent with Si clusters in Li.

Recently nitrogen-hydrogen compounds have successfully been applied as co-catalysts for mild conditions ammonia synthesis. Ca₂NH was shown to act as a H₂ sink during reaction, with H atoms from its lattice being incorporated into the NH₃(g) product. Thus the ionic transport and diffusion properties of the N–H co-catalyst are fundamentally important to understanding and developing such syntheses. Here we show hydride ion conduction in these materials. Two distinct calcium nitride-hydride Ca₂NH phases, prepared via different synthetic paths are found to show dramatically different properties. One phase (β) shows fast hydride ionic conduction properties (0.08 S/cm at 600 °C), on a par with the best binary ionic hydrides and 10 times higher than CaH₂, whilst the other (α) is 100 times less conductive. An in situ combined analysis techniques reveals that the effective β-phase conducts ions via a vacancy-mediated phenomenon in which the charge carrier concentration is dependent on the ion concentration in the secondary site and by extension the vacancy concentration in the main site.