Extended Ni(III) Oxyhalide Perovskite Derivatives: Sr2NiO3X (X = F, Cl)

High Conductivity and Diffusion Mechanism of Oxide Ions in Triple Fluorite-Like Layers of Oxyhalides

Oxide ion conductors are attractive materials because of their wide range of applications, such as solid oxide fuel cells. Oxide ion conduction in oxyhalides (compounds containing both oxide ions and halide ions) is rare. In the present work, we found that Sillén oxychlorides, Bi2-xTexLuO4+x/2Cl (x = 0, 0.1, and 0.2), show high oxide ion conductivity. The bulk conductivity of Bi1.9Te0.1LuO4.05Cl reaches 10-2 S cm-1 at 431 °C, which is much lower than 644 °C of yttria-stabilized zirconia (YSZ) and 534 °C of La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM). Thanks to the low activation energy, Bi1.9Te0.1LuO4.05Cl exhibits a high bulk conductivity of 1.5 × 10-3 S cm-1 even at a low temperature of 310 °C, which is 204 times higher than that of YSZ. The low activation energy is attributed to the interstitialcy oxide ion diffusion in the triple fluorite-like layer, as evidenced by neutron diffraction experiments (Rietveld and neutron scattering length density analyses), bond valence-based energy calculations, static DFT calculations, and ab initio molecular dynamics simulations. The electrical conductivity of Bi1.9Te0.1LuO4.05Cl is almost independent of the oxygen partial pressure from 10-18 to 10-4 atm at 431 °C, indicating the electrolyte domain. Bi1.9Te0.1LuO4.05Cl also exhibits high chemical stability under a CO2 flow and ambient air at 400 °C. The oxide ion conduction due to the two-dimensional interstitialcy diffusion is considered to be common in Sillén oxyhalides with triple fluorite-like layers, such as Bi1.9Te0.1RO4.05Cl (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) and Bi6-2xTe2xO8+xBr2 (x = 0.1, 0.5). The present study opens a new field of materials chemistry: oxide ion-conducting Sillén oxyhalides with triple fluorite-like layers.

Pinalites: Optical Properties and Quantum Magnetism of Heteroanionic A3MO5X2 Compounds

Heteroanionic compounds, which contain two or more types of anions, have emerged as a promising class of materials with diverse properties and functionalities. In this paper, I review the experimental findings on Ca3ReO5Cl2 and related compounds that exhibit remarkable pleochroism and novel quantum magnetism. I discuss how the heteroanionic coordination affects the optical and magnetic properties by modulating the d-orbital states of the transition metal ions. Subsequently, I compare these materials with other heteroanionic and monoanionic compounds and highlight the potential of A3MO5X2 materials for future exploration of materials and phenomena.

Ca2MnO3X (X = Cl, Br) Oxyhalides with 1-Dimensional Ferromagnetic Chains of Square-Planar S = 2 Mn3

Mixed anion oxyhalides with the formula Ca2MnO3X (X = Cl, Br) are synthesized using solid-state reaction methods. These two materials crystallize in a novel structure type due to the small ionic radius of Ca and the strong Jahn-Teller effect of Mn3+. The resulting structure (space group Cmcm) contains one-dimensional chains of MnO4 square planes, with an angle of ∼120° between neighboring planes. At low temperatures, the two materials adopt magnetic arrangements, with ferromagnetic chains coupled antiferromagnetically. On applying a magnetic field, both materials experience spin-flop transitions.

Synthesis of Fluoride-Substituted Layered Perovskites ZnMoO4 with an Enhanced Photocatalytic Activity

Oxyfluorides possess considerable attention for their multiple excellent properties, but the conventional high-temperature solid-state syntheses have seen bottlenecks in the synthesis of new compounds. Herein, we report a novel layered oxyfluoride ZnMoO4:F, which is prepared by a facile hydrothermal method using ZnF2 as the fluoride source. The fluoride anions are successfully introduced into the oxygen sublattice, which is confirmed by a combined analysis using XRD, STEM, and TGA techniques. The as-synthesized ZnMoO4:F has an absorption edge at around 550 nm, indicating a red shift of Eg to the visible region compared to the oxide counterpart. The layered oxyfluoride exhibits an enhanced photocatalytic active for hydrogen evolution under simulated sunlight (λ > 350 nm), and the activity of ZnMoO4:F (651.9 μmol g-1) was 2 times higher than that of ZnMoO4 (309.7 μmol g-1). Further electrochemical analysis has shown that the conduction band position plays a critical role in the high performances of ZnMoO4:F. This work sheds new light on the future design and synthesis of novel fluoride-doped materials for photocatalysis applications.

Synthesis and Characterization of Oxide Chloride Sr2VO3Cl, a Layered S = 1 Compound

The mixed-anion compound with composition Sr₂VO₃Cl has been synthesized for the first time, using the conventional high-temperature solid-state synthesis technique in a closed silica ampule under inert conditions. This compound belongs to the known Sr₂ TmO₃Cl (Tm = Sc, Mn, Fe, Co, Ni) family, but with Tm = V. All homologues within this family can be described with the tetragonal space group P4/nmm (No. 129); from a Rietveld refinement of powder X-ray diffraction data on the Tm = V homologue, the unit cell parameters were determined to a = 3.95974(8) and c = 14.0660(4) Å, and the atomic parameters in the crystal structure could be estimated. The synthesized powder is black, implying that the compound is a semiconductor. The magnetic investigations suggest that Sr₂VO₃Cl is a paramagnet at high temperatures, exhibiting a μ_eff = 2.0 μ_B V^-1 and antiferromagnetic (AFM) interactions between the magnetic vanadium spins (θ_CW = -50 K), in line with the V-O-V advantageous super-exchange paths in the V-O layers. Specific heat capacity studies indicate two small anomalies around 5 and 35 K, which however are not associated with long-range magnetic ordering. ³⁵Cl ss-NMR investigations suggest a slow spin freezing below 4.2 K resulting in a glassy-like spin ground state.

Synthesis and H- conductivity of a new oxyhydride Ba2YHO3 with anion-ordered rock-salt layers

A new layered perovskite oxyhydride Ba₂YHO₃ was synthesized via high pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that in Ba₂YHO₃ the ordered H^- and O^2- anions form [Ba₂H₂] rock-salt layers wherein H^- conduction is allowed. The conductivity reached 0.1 mS cm^-1 at 350 °C, which is higher than that of isostructural Ba₂ScHO₃ with anion-disordered [Ba₂HO] layers.

Predicting the Structure Stability of Layered Heteroanionic Materials Exhibiting Anion Order

We report a workflow for heteroanionic materials discovery using Pauling's second rule to filter for and predict new candidate materials for synthesis with reduced computational overhead. Using oxyfluoride and oxynitride n = 1 Ruddlesden-Popper compounds as a use-case, we show that a minimization scheme based on the global instability index (GII) efficiently filters up to 50% of highly unstable candidate compositions based on crystal-chemistry grounds. We then validate the minimization scheme using density functional theory (DFT) calculations and find that unexpectedly the GII of stable heteroanionic materials is higher than that of homoanionic oxides owing to significant charge redistribution in compounds containing more than one anionic species. Using this workflow, we predict Sr₂AlO₃F to be stable and describe our attempts to synthesize a phase-pure material.

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide-based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent-bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide-only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion-engineering principles in heteroanionic materials are reviewed.

Ba2ScHO3: H- Conductive Layered Oxyhydride with H- Site Selectivity

Hydride (H^-) conduction is a new frontier related to hydrogen transport in solids. Here, a new H^- conductive oxyhydride Ba₂ScHO₃ was successfully synthesized using a high-pressure technique. Powder X-ray and neutron diffraction experiments investigated the fact that Ba₂ScHO₃ adopts a K₂NiF₄-type structure with H^- ions preferentially occupying the apical sites, as supported by theoretical calculations. Electrochemical impedance spectra showed that Ba₂ScHO₃ exhibited H^- conduction and a conductivity of 5.2 × 10^-6 S cm^-1 at 300 °C. This value is much higher than that of BaScO₂H, which has an ideal perovskite structure, suggesting the advantage of layered structures for H^- conduction. Tuning site selectivity of H^- ions in layered oxyhydrides might be a promising strategy for designing fast H^- conductors applicable for novel electrochemical devices.

Synthesis, Crystal Structure, and Optical Properties of Layered Perovskite Scandium Oxychlorides: Sr2ScO3Cl, Sr3Sc2O5Cl2, and Ba3Sc2O5Cl2

We report the successful synthesis of three new Ruddlesden-Popper-type scandium oxychloride perovskites, Sr₂ScO₃Cl, Sr₃Sc₂O₅Cl₂, and Ba₃Sc₂O₅Cl₂, by conventional solid-state reaction. Small single crystals of Sr₂ScO₃Cl were obtained by a self-flux method, and the crystal structure was determined to belong to the tetragonal P4/ nmm space group ( a = 4.08066(14) Å, c = 14.1115(8) Å) by X-ray diffraction analysis. The scandium center forms a ScO₅Cl octahedron with ordered apical oxygen and chlorine anions. The scandium cation, however, is shifted from the position of the octahedral center toward the apical oxygen anion, such that the coordination geometry of the Sc cation can be effectively viewed as an ScO₅ pyramid. These structural features in the oxychloride are different from those of octahedral ScO₅F coordinated with a partial O/F anion order at the apical sites in the oxyfluoride Sr₂ScO₃F. Rietveld refinements of the neutron powder diffraction data of Sr₃Sc₂O₅Cl₂ ( I4/ mmm: a = 4.107982(5) Å, c = 23.58454(7) Å) and Ba₃Sc₂O₅Cl₂ ( I4/ mmm: a = 4.206920(5) Å, c = 24.54386(6) Å) reveal the presence of pseudo ScO₅ pyramids with the Cl anion being distant from the scandium cation, which is similar to the Sc-centered coordination geometry in Sr₂ScO₃Cl with the exception that the ScO₅ pyramids form double layers by sharing the apical oxygen. Density functional calculations on Sr₂ScO₃Cl indicate the strong covalency of the Sc-O bonds but almost nonbonding interaction between Sc and Cl ions.

Strontium cobalt oxychlorides: enhanced electrocatalysts for oxygen reduction and evolution reactions

Cobalt-based layered perovskite oxychlorides Sr₂CoO₃Cl and Sr₃Co₂O₅Cl₂ exhibit high oxygen electrochemical activity compared to conventional lanthanum cobalt-based perovskite oxides.

Pressure-Driven Spin Crossover Involving Polyhedral Transformation in Layered Perovskite Cobalt Oxyfluoride

We report a novel pressure-driven spin crossover in layered cobalt oxyfluoride Sr₂CoO₃F with a distorted CoO₅ square pyramid loosely bound with a fluoride ion. Upon increasing pressure, the spin state of the Co(III) cation gradually changes from a high spin state (S = 2) to a low spin state (S = 0) accompanied by a anomalously large volume contraction (bulk modulus, 76.8(5) GPa). The spin state change occurs on the CoO₅ pyramid in a wide pressure range, but the concomitant gradual shrinkage of the Co–F bond length with pressure gives rise to a polyhedral transformation to the CoO₅F octahedron without a structural phase transition, leading to the full conversion to the LS state at 12 GPa. The present results provide new effective strategy to fine-tune electronic properties of mixed anion systems by controlling the covalency in metal-ligand bonds under pressure.

MnTaO2N: polar LiNbO3-type oxynitride with a helical spin order

The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O(2-) and N(3-) anions. MnTaO2N with an "orbital-inactive" Mn(2+) ion (d(5); S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O(2-)/N(3-) species that results in a distribution of (super)-superexchange interactions.

New members of layered oxychloride perovskites with square planar coordination: Sr2MO2Cl2 (M = Mn, Ni) and Ba2PdO2Cl2

We have demonstrated high-pressure syntheses of Ruddlesden–Popper type layered oxychloride perovskites, Sr₂MnO₂Cl₂, Sr₂NiO₂Cl₂ and Ba₂PdO₂Cl₂, with a square planar coordination around the transition metal centres.