Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batteries

Kröhnkite-Type Na₂Fe(SO₄)₂·2H₂O as a Novel 3.25 V Insertion Compound for Na-Ion Batterie

Hydration Mechanisms and Proton Conduction in the Mixed Ionic–Electronic Conductors Ba₄Nb₂O₉ and Ba₄Ta₂O₉

We studied the behavior of hydrogen in the mixed ionic–electronic conductors γ-Ba₄Nb₂O₉ and 6H-Ba₄Ta₂O₉ using a combination of experimental (neutron diffraction and inelastic neutron scattering) and computational (ab initio molecular dynamics) methods. Although these compounds have isostructural low-temperature polymorphs, they adopt distinct forms in the high-temperature conducting regime. We show here that they also have distinct mechanisms for hydration and ionic conduction. Hydration of γ-Ba₄Nb₂O₉ is localized to 2-D layers in the structure that contain a 1:1 ratio of isolated but adjacent NbO₄ and NbO₅ polyhedra. OH^– and H⁺ ions combine with two polyhedra, respectively, to form complete layers of NbO₄OH polyhedra, giving rise to a stoichiometric hydrated form γ-III-Ba₄Nb₂O₉·¹/₃H₂O. Protons then diffuse through these 2-D layers by “hopping” between oxygen atoms on adjacent polyhedra. In the case of 6H-Ba₄Ta₂O₉, hydration occurs by intercalating intact water molecules into the structure up to a maximum of ∼0.375 H₂O per formula unit. This explains the unusual local and long-range structural distortions in the hydrated form observed by neutron diffraction. Diffusion then occurs by water molecules moving between neighboring symmetry equivalent positions. These fundamentally different hydration and proton conduction mechanisms explain why 6H-Ba₄Ta₂O₉ has the less well-defined and higher maximum water content, while γ-Ba₄Nb₂O₉ has the higher proton conductivity

A New <i>n</i> = 4 Layered Ruddlesden–Popper Phase K_2.5Bi_2.5Ti₄O₁₃ Showing Stoichiometric Hydration

A new bismuth-containing layered perovskite of the Ruddlesden–Popper type, K_2.5Bi_2.5Ti₄O₁₃, has been prepared by solid-state synthesis. It has been shown to hydrate to form stoichiometric K_2.5Bi_2.5Ti₄O₁₃·H₂O. Diffraction data show that the structure consists of a quadruple-stacked (<i>n</i> = 4) perovskite layer, with potassium ions occupying the rock salt layer and its next-nearest A site. The hydrated sample was shown to remove the offset between stacked perovskite layers relative to the dehydrated sample. Computational methods show that the hydrated phase consists of intact H₂O molecules in a vertical “pillared” arrangement bridging across the interlayer space. Rotations of H₂O molecules about the <i>c</i> axis were evident in molecular dynamic calculations, which increased in rotation angle with increasing temperature. In situ diffraction data for the dehydrated phase point to a broad structural phase transition from orthorhombic to tetragonal at ∼600 °C. The relative bismuth-rich composition in the perovskite block results in a higher transition temperature compared to related perovskite structures. Water makes a significant contribution to the dielectric constant, which disappears after dehydration

A New <i>n</i> = 4 Layered Ruddlesden–Popper Phase K_2.5Bi_2.5Ti₄O₁₃ Showing Stoichiometric Hydration

A new bismuth-containing layered perovskite of the Ruddlesden–Popper type, K_2.5Bi_2.5Ti₄O₁₃, has been prepared by solid-state synthesis. It has been shown to hydrate to form stoichiometric K_2.5Bi_2.5Ti₄O₁₃·H₂O. Diffraction data show that the structure consists of a quadruple-stacked (<i>n</i> = 4) perovskite layer, with potassium ions occupying the rock salt layer and its next-nearest A site. The hydrated sample was shown to remove the offset between stacked perovskite layers relative to the dehydrated sample. Computational methods show that the hydrated phase consists of intact H₂O molecules in a vertical “pillared” arrangement bridging across the interlayer space. Rotations of H₂O molecules about the <i>c</i> axis were evident in molecular dynamic calculations, which increased in rotation angle with increasing temperature. In situ diffraction data for the dehydrated phase point to a broad structural phase transition from orthorhombic to tetragonal at ∼600 °C. The relative bismuth-rich composition in the perovskite block results in a higher transition temperature compared to related perovskite structures. Water makes a significant contribution to the dielectric constant, which disappears after dehydration

Hydration Mechanisms and Proton Conduction in the Mixed Ionic–Electronic Conductors Ba₄Nb₂O₉ and Ba₄Ta₂O₉

We studied the behavior of hydrogen in the mixed ionic–electronic conductors γ-Ba₄Nb₂O₉ and 6H-Ba₄Ta₂O₉ using a combination of experimental (neutron diffraction and inelastic neutron scattering) and computational (ab initio molecular dynamics) methods. Although these compounds have isostructural low-temperature polymorphs, they adopt distinct forms in the high-temperature conducting regime. We show here that they also have distinct mechanisms for hydration and ionic conduction. Hydration of γ-Ba₄Nb₂O₉ is localized to 2-D layers in the structure that contain a 1:1 ratio of isolated but adjacent NbO₄ and NbO₅ polyhedra. OH^– and H⁺ ions combine with two polyhedra, respectively, to form complete layers of NbO₄OH polyhedra, giving rise to a stoichiometric hydrated form γ-III-Ba₄Nb₂O₉·¹/₃H₂O. Protons then diffuse through these 2-D layers by “hopping” between oxygen atoms on adjacent polyhedra. In the case of 6H-Ba₄Ta₂O₉, hydration occurs by intercalating intact water molecules into the structure up to a maximum of ∼0.375 H₂O per formula unit. This explains the unusual local and long-range structural distortions in the hydrated form observed by neutron diffraction. Diffusion then occurs by water molecules moving between neighboring symmetry equivalent positions. These fundamentally different hydration and proton conduction mechanisms explain why 6H-Ba₄Ta₂O₉ has the less well-defined and higher maximum water content, while γ-Ba₄Nb₂O₉ has the higher proton conductivity

YCa₃(CrO)₃(BO₃)₄: A Cr³⁺ Kagomé Lattice Compound Showing No Magnetic Order down to 2 K

We report a new gaudefroyite-type compound YCa₃(CrO)₃(BO₃)₄, in which Cr³⁺ ions (3d³, <i>S</i> = 3/2) form an undistorted kagomé lattice. Using a flux agent, the synthesis was significantly accelerated with the typical calcining time reduced from more than 2 weeks to 2 d. The structure of YCa₃(CrO)₃­(BO₃)₄ was determined by combined Rietveld refinements against X-ray and neutron diffraction data. Symmetry distortion refinement starting from a disordered YCa₃(MnO)₃­(BO₃)₄ model was applied to avoid overparameterization. There are two ordering models, namely, K2–1 and K2–2, with the space groups <i>P</i>6₃ (No. 173) and <i>P</i>3̅ (No. 147), respectively, that differ in the [BO₃] ordering between different channels (in-phase or out-of-phase). Both models give similarly good fits to the diffraction data. YCa₃(CrO)₃­(BO₃)₄ is an insulator with the major band gap at <i>E</i>_g = 1.65 eV and a second transition at 1.78 eV. Magnetically, YCa₃­(CrO)₃(BO₃)₄ is dominated by anti-ferromagnetic exchange along edge-sharing CrO₆ octahedral chains perpendicular to the kagomé planes, with Θ ≈ −120 K and μ_eff ≈ 3.92 μ_B. The compound shows no spin ordering or freezing down to at least 2 K

Competing Magnetic Interactions and the Role of Unpaired 4<i>f</i> Electrons in Oxygen-Deficient Perovskites Ba₃<i>R</i>Fe₂O_7.5 (<i>R</i> = Y, Dy)

Oxygen-deficient perovskite compounds with the general formula Ba3RFe2O7.5 present a good opportunity to study competing magnetic interactions between Fe3+ 3d cations with and without the involvement of unpaired 4f electrons on R3+ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R3+ = Y3+ (non-magnetic) and Dy3+ (4f9). They both adopt complex long-range ordered antiferromagnetic structures below TN = 6.6 and 14.5 K, respectively, with the same magnetic space group Ca2/c (BNS #15.91). However, the dominant influence of f-electron magnetism is clear in temperature dependence and differences between the size of the ordered moments on the two crystallographically independent Fe sites, one of which is enhanced by R–O–Fe superexchange in the Dy compound, while the other is frustrated by it. The Dy compound also shows evidence for temperature- and field-dependent transitions with hysteresis, indicating a field-induced ferromagnetic component below TN

Synthetic, Structural, and Electrochemical Study of Monoclinic Na₄Ti₅O₁₂ as a Sodium-Ion Battery Anode Material

The monoclinic phase of Na₄Ti₅O₁₂ (M-Na₄Ti₅O₁₂) has been investigated as a potential sodium-ion battery anode material. In contrast to the previously investigated trigonal phase (T-Na₄Ti₅O₁₂), M-Na₄Ti₅O₁₂ has continuous two-dimensional (2D) channels with partially occupied Na sites, providing broader pathways and more space for the intercalation of excess sodium. Electrochemical measurements show that it exhibits a comparable or higher reversible capacity than T-Na₄Ti₅O₁₂. Neutron powder diffraction reveals the preferred sites and occupancies of the excess sodium. <i>In situ</i> synchrotron X-ray diffraction under electrochemical cycling shows that the crystal lattice undergoes strongly anisotropic volume changes during cycling

Synthetic, Structural, and Electrochemical Study of Monoclinic Na₄Ti₅O₁₂ as a Sodium-Ion Battery Anode Material

The monoclinic phase of Na₄Ti₅O₁₂ (M-Na₄Ti₅O₁₂) has been investigated as a potential sodium-ion battery anode material. In contrast to the previously investigated trigonal phase (T-Na₄Ti₅O₁₂), M-Na₄Ti₅O₁₂ has continuous two-dimensional (2D) channels with partially occupied Na sites, providing broader pathways and more space for the intercalation of excess sodium. Electrochemical measurements show that it exhibits a comparable or higher reversible capacity than T-Na₄Ti₅O₁₂. Neutron powder diffraction reveals the preferred sites and occupancies of the excess sodium. <i>In situ</i> synchrotron X-ray diffraction under electrochemical cycling shows that the crystal lattice undergoes strongly anisotropic volume changes during cycling

Long-Range-Ordered Coexistence of 4‑, 5‑, and 6‑Coordinate Niobium in the Mixed Ionic-Electronic Conductor γ‑Ba₄Nb₂O₉

In a study combining high-resolution single-crystal neutron diffraction and solid-state nuclear magnetic resonance, the mixed ionic-electronic conductor γ-Ba₄Nb₂O₉ is shown to have a unique structure type, incorporating niobium in 4-, 5-, and 6-coordinate environments. The 4- and 5-coordinate niobium tetrahedra and trigonal bipyrimids exist in discrete layers, within and among which their orientations vary systematically to form a complex superstructure. Through analysis and comparison of data obtained from hydrated versus dehydrated samples, a mechanism is proposed for the ready hydration of the material by atmospheric water. This mechanism, and the resulting hydrated structure, help explain the high protonic and oxide ionic conductivity of γ-Ba₄Nb₂O₉