1:1:1 Triple-Cation B‑Site-Ordered and Oxygen-Deficient Perovskite Ca₄GaNbO₈: A Member of a Family of Anion-Vacancy-Based Cation-Ordered Complex Perovskites

Exploration of the Ca–Ga–Nb–O phase diagram by solid-state reaction in air led to isolation of Ca₄GaNbO₈. The crystal structure was determined ab initio by synchrotron X-ray and high-resolution neutron powder diffraction. Ca₄GaNbO₈ adopts a heavily distorted oxygen-deficient perovskite structure with the rare feature of complete ordering of the three B-site cations, driven by their distinct chemistries. One of the calcium cations occupies a distorted octahedral cavity and together with tetrahedrally coordinated Ga and octahedrally coordinated Nb is considered as a B-site cation in the ABO_3–<i>x</i> perovskite. This interpretation of the structure reveals Ca₄GaNbO₈ is part of a family of B-site and vacancy-ordered perovskites related to mineral structures. The anion-vacancy ordering pattern in Ca₄GaNbO₈ is driven by the coordination preferences of the three structurally distinct cations and correlated with the ordering of each cation on a distinct site. Alternating current impedance spectra show Ca₄GaNbO₈ is insulating (bulk conductivity 10^–5–10^–7 S·cm^–1) over the measured temperature range 550–950 °C with an activation energy of 1.10(3) eV

1:1:1 Triple-Cation B‑Site-Ordered and Oxygen-Deficient Perovskite Ca₄GaNbO₈: A Member of a Family of Anion-Vacancy-Based Cation-Ordered Complex Perovskites

Exploration of the Ca–Ga–Nb–O phase diagram by solid-state reaction in air led to isolation of Ca₄GaNbO₈. The crystal structure was determined ab initio by synchrotron X-ray and high-resolution neutron powder diffraction. Ca₄GaNbO₈ adopts a heavily distorted oxygen-deficient perovskite structure with the rare feature of complete ordering of the three B-site cations, driven by their distinct chemistries. One of the calcium cations occupies a distorted octahedral cavity and together with tetrahedrally coordinated Ga and octahedrally coordinated Nb is considered as a B-site cation in the ABO_3–<i>x</i> perovskite. This interpretation of the structure reveals Ca₄GaNbO₈ is part of a family of B-site and vacancy-ordered perovskites related to mineral structures. The anion-vacancy ordering pattern in Ca₄GaNbO₈ is driven by the coordination preferences of the three structurally distinct cations and correlated with the ordering of each cation on a distinct site. Alternating current impedance spectra show Ca₄GaNbO₈ is insulating (bulk conductivity 10^–5–10^–7 S·cm^–1) over the measured temperature range 550–950 °C with an activation energy of 1.10(3) eV

BaFe₉LiO₁₅: A New Layered Antiferromagnetic Ferrite

The new Fe³⁺ oxide BaFe₉LiO₁₅ is isostructural with the magnetically frustrated material BaV₁₀O₁₅, adopting a structure based on the stacking of close-packed pure oxide and BaO₇ layers. Neutron diffraction and Mössbauer spectroscopy shows that BaFe₉LiO₁₅ is long-range antiferromagnetically ordered with a Néel temperature of 460 K. The magnetic ordering of antiferromagnetically coupled ferromagnetic planes is stabilized by 90° and 180° superexchange interactions between the Fe³⁺ cations that supersede the frustrated in-plane direct exchange observed in t_2g-only systems

Comprehensive Study of DNA Binding on Iron(II,III) Oxide Nanoparticles with a Positively Charged Polyamine Three-Dimensional Coating

Iron (II,III) oxide Fe₃O₄ nanoparticles (25 and 50 nm NPs) are grafted with amine groups through silanization in order to generate a positively charged coating for binding negatively charged species including DNA molecules. The spatial nature of the coating changes from a 2-D-functionalized surface (monoamines) through a layer of amine oligomers (diethylenetriamine or DETA, about 1 nm in length) to a 3-D layer of polyamine (polyethyleneimine or PEI, thickness ≥3.5 nm). These Fe₃O₄–PEI NPs were prepared by binding short-chain PEI polymers to the iodopropyl groups grafted on the NP surface. In this work, the surface charge density, or zeta potential, of the nanoparticles is found not to be the only factor influencing the DNA binding capacity, which also seems not to be affected by their buffering capacity profile in the range of pH 4–10. This study also allows the investigation of this 3-D effect on the surface of a nanoparticle as opposed to conventional 2-D amine functionalization. The flexibility of the PEI coating, which consists of only 1, 2, and 3° amines, on the nanoparticle surface has a significant influence on the overall DNA binding capacity and the binding efficiency (or N/P ratio). These polyamine-functionalized nanoparticles can be used in the purification of biomolecules and the delivery of drugs and large biomolecules

Local Structure of a Pure Bi <i>A</i> Site Polar Perovskite Revealed by Pair Distribution Function Analysis and Reverse Monte Carlo Modeling: Correlated Off-Axis Displacements in a Rhombohedral Material

Perovskite oxides with Bi³⁺ on the <i>A</i> site are of interest as candidate replacements for lead-based piezoelectric ceramics. Current understanding of the chemical factors permitting the synthesis of ambient-pressure-stable perovskite oxides with Bi³⁺ on the <i>A</i> site is limited to information derived from average structures. The local structure of the lead-free ferroelectric perovskite Bi­(Ti_3/8Fe_2/8Mg_3/8)­O₃ is studied by reverse Monte Carlo (RMC) modeling of neutron scattering data. The resultant model is consistent with the structure derived from diffraction but reveals key extra structural features due to correlated local displacements that are inaccessible from the average unit cell. The resulting structural picture emphasizes the need to combine symmetry-averaged long-range and local analysis of the structures of compositionally complex, substitutionally disordered functional materials. Local correlation of the off-axis displacements of the <i>A</i> site cation produces monoclinic domains consistent with the existence of displacement directions other than R (⟨111⟩_p) or T (⟨100⟩_p). The Bi displacements are correlated ferroelectrically both in the polar direction and orthogonal to it, providing evidence of the presence of monoclinic domains. The octahedral cation environments reveal distinct differences in the coordination geometry of the different <i>B</i> site metal ions. The local nature of these deviations and correlations makes them inaccessible to long-range averaged techniques. The resulting local structure information provides a new understanding of the stability of pure Bi ³⁺ <i>A</i> site perovskite oxides

New 8-Layer Twinned Hexagonal Perovskite Microwave Dielectric Ceramics Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄

An 8-layer B-site deficient twinned hexagonal perovskite Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO₃ layers stacked by a sequence of (ccch)₂, where c and h refer to cubic and hexagonal BaO₃ layers, respectively. The Ba₈Ga_4–<i>x</i>Ta_{4+0.6<i>x</i>}O₂₄ ceramic materials exhibit composition-independent dielectric permittivity ε_r ≈ 29, improved <i>Q</i><i>f</i> value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency τ_f from negative to positive. An optimum microwave dielectric performance was achieved for Ba₈Ga_0.8Ta_5.92O₂₄: <i>Q</i><i>f</i> ≈ 29 000 GHz and τ_f ≈ 11 ppm/°C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data

Local Crystal Structure of Antiferroelectric Bi₂Mn_4/3Ni_2/3O₆ in Commensurate and Incommensurate Phases Described by Pair Distribution Function (PDF) and Reverse Monte Carlo (RMC) Modeling

The functional properties of materials can arise from local structural features that are not well determined or described by crystallographic methods based on long-range average structural models. The room temperature (RT) structure of the Bi perovskite Bi₂Mn_4/3Ni_2/3O₆ has previously been modeled as a locally polar structure where polarization is suppressed by a long-range incommensurate antiferroelectric modulation. In this study we investigate the short-range local structure of Bi₂Mn_4/3Ni_2/3O₆, determined through reverse Monte Carlo (RMC) modeling of neutron total scattering data, and compare the results with the long-range incommensurate structure description. While the incommensurate structure has equivalent B site environments for Mn and Ni, the local structure displays a significantly Jahn–Teller distorted environment for Mn³⁺. The local structure displays the rock-salt-type Mn/Ni ordering of the related Bi₂MnNiO₆ high pressure phase, as opposed to Mn/Ni clustering observed in the long-range average incommensurate model. RMC modeling reveals short-range ferroelectric correlations between Bi³⁺ cations, giving rise to polar regions that are quantified for the first time as existing within a distance of approximately 12 Å. These local correlations persist in the commensurate high temperature (HT) phase, where the long-range average structure is nonpolar. The local structure thus provides information about cation ordering and B site structural flexibility that may stabilize Bi³⁺ on the A site of the perovskite structure and reveals the extent of the local polar regions created by this cation

Visible Light Photo-oxidation of Model Pollutants Using CaCu₃Ti₄O₁₂: An Experimental and Theoretical Study of Optical Properties, Electronic Structure, and Selectivity

Charge transfer between metal ions occupying distinct crystallographic sublattices in an ordered material is a strategy to confer visible light absorption on complex oxides to generate potentially catalytically active electron and hole charge carriers. CaCu₃Ti₄O₁₂ has distinct octahedral Ti⁴⁺ and square planar Cu²⁺ sites and is thus a candidate material for this approach. The sol−gel synthesis of high surface area CaCu₃Ti₄O₁₂ and investigation of its optical absorption and photocatalytic reactivity with model pollutants are reported. Two gaps of 2.21 and 1.39 eV are observed in the visible region. These absorptions are explained by LSDA+U electronic structure calculations, including electron correlation on the Cu sites, as arising from transitions from a Cu-hybridized O 2p-derived valence band to localized empty states on Cu (attributed to the isolation of CuO₄ units within the structure of CaCu₃Ti₄O₁₂) and to a Ti-based conduction band. The resulting charge carriers produce selective visible light photodegradation of 4-chlorophenol (monitored by mass spectrometry) by Pt-loaded CaCu₃Ti₄O₁₂ which is attributed to the chemical nature of the photogenerated charge carriers and has a quantum yield comparable with commercial visible light photocatalysts

Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li_6+<i>x</i>P_1–<i>x</i>Si_<i>x</i>O₅Cl

Argyrodite is a key structure type for ion-transporting materials. Oxide argyrodites are largely unexplored despite sulfide argyrodites being a leading family of solid-state lithium-ion conductors, in which the control of lithium distribution over a wide range of available sites strongly influences the conductivity. We present a new cubic Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic (P213) structure at room temperature, undergoing a transition at 473 K to a Li+ site disordered F4̅3m structure, consistent with the symmetry adopted by superionic sulfide argyrodites. Four different Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously unreported for Li-containing argyrodites. The disordered F4̅3m structure is stabilized to room temperature via substitution of Si4+ with P5+ in Li6+xP1–xSixO5Cl (0.3 x < 0.85) solid solution. The resulting delocalization of Li+ sites leads to a maximum ionic conductivity of 1.82(1) × 10–6 S cm–1 at x = 0.75, which is 3 orders of magnitude higher than the conductivities reported previously for oxide argyrodites. The variation of ionic conductivity with composition in Li6+xP1–xSixO5Cl is directly connected to structural changes occurring within the Li+ sublattice. These materials present superior atmospheric stability over analogous sulfide argyrodites and are stable against Li metal. The ability to control the ionic conductivity through structure and composition emphasizes the advances that can be made with further research in the open field of oxide argyrodites

Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide

Complex transition-metal oxides are important functional materials in areas such as energy and information storage. The cubic ABO₃ perovskite is an archetypal example of this class, formed by the occupation of small octahedral B-sites within an AO₃ network defined by larger A cations. We show that introduction of chemically mismatched octahedral cations into a cubic perovskite oxide parent phase modifies structure and composition beyond the unit cell length scale on the B sublattice alone. This affords an endotaxial nanocomposite of two cubic perovskite phases with distinct properties. These locally B-site cation-ordered and -disordered phases share a single AO₃ network and have enhanced stability against the formation of a competing hexagonal structure over the single-phase parent. Synergic integration of the distinct properties of these phases by the coherent interfaces of the composite produces solid oxide fuel cell cathode performance superior to that expected from the component phases in isolation