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

La_1+<i>x</i>Ba_1–<i>x</i>Ga₃O_{7+0.5<i>x</i>} Oxide Ion Conductor: Cationic Size Effect on the Interstitial Oxide Ion Conductivity in Gallate Melilites

Substitution of La³⁺ for Ba²⁺ in LaBaGa₃O₇ melilite yields a new interstitial-oxide-ion conducting La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} solid solution, which only extends up to <i>x</i> = 0.35, giving a maximum interstitial oxygen content allowed in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} as about half of those allowed in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. La_1.35Ba_0.65­Ga₃O_7.175 ceramic displays bulk conductivity ∼1.9 × 10^–3 S/cm at 600 °C, which is lower than those of La_1.35(Sr/Ca)_0.65­Ga₃O_7.175, showing the reduced mobility for the oxygen interstitials in La_1+<i>x</i>Ba_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} than in La_1+<i>x</i>(Sr/Ca)_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>}. Rietveld analysis of neutron powder diffraction data reveals that the oxygen interstitials in La_1.35Ba_0.65Ga₃O_7.175 are located within the pentagonal tunnels at the Ga level between two La/Ba cations along the <i>c</i>-axis and stabilized via incorporating into the bonding environment of a three-linked GaO₄ among the five GaO₄ tetrahedra forming the pentagonal tunnels, similar to the Sr and Ca counterparts. Both static lattice atomistic simulation and density functional theory calculation show that LaBaGa₃O₇ has the largest formation energy for oxygen interstitial defects among La_1+<i>x</i>M_1–<i>x</i>­Ga₃O_{7+0.5<i>x</i>} (M = Ba, Sr, Ca), consistent with the large Ba²⁺ cations favoring interstitial oxygen defects in melilite less than the small cations Sr²⁺ and Ca²⁺. The cationic-size control of the ability to accommodate the oxygen interstitials and maintain high mobility for the oxygen interstitials in La_1+<i>x</i>M_1–<i>x</i>Ga₃O_7+0.5x (M = Ba, Sr, Ca) gallate melilites is understood in terms of local structural relaxation to accommodate and transport the oxygen interstitials. The accommodation and migration of the interstitials in the melilite structure require the tunnel-cations being able to adapt to the synergic size expansion for the interstitial-containing tunnel and contraction for the tunnels neighboring the interstitial-containing tunnel and continuous tunnel-size expansion and contraction. However, the large oxygen bonding separation requirement of the large Ba²⁺ along the tunnel not only suppresses the ability to accommodate the interstitials in the tunnels neighboring the Ba²⁺-containing tunnel but also reduces the mobility of the oxygen interstitials among the pentagonal tunnels

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

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

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 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

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

Bi₄O₄Cu_1.7Se_2.7Cl_0.3: Intergrowth of BiOCuSe and Bi₂O₂Se Stabilized by the Addition of a Third Anion

Layered two-anion compounds are of interest for their diverse electronic properties. The modular nature of their layered structures offers opportunities for the construction of complex stackings used to introduce or tune functionality, but the accessible layer combinations are limited by the crystal chemistries of the available anions. We present a layered three-anion material, Bi₄O₄Cu_1.7Se_2.7Cl_0.3, which adopts a new structure type composed of alternately stacked BiOCuSe and Bi₂O₂Se-like units. This structure is accessed by inclusion of three chemically distinct anions, which are accommodated by aliovalently substituted Bi₂O₂Se_0.7Cl_0.3 blocks coupled to Cu-deficient Bi₂O₂Cu_1.7Se₂ blocks, producing a formal charge modulation along the stacking direction. The hypothetical parent phase Bi₄O₄Cu₂Se₃ is unstable with respect to its charge-neutral stoichiometric building blocks. The complex layer stacking confers excellent thermal properties upon Bi₄O₄Cu_1.7Se_2.7Cl_0.3: a room-temperature thermal conductivity (κ) of 0.4(1) W/mK was measured on a pellet with preferred crystallite orientation along the stacking axis, with perpendicular measurement indicating it is also highly anisotropic. This κ value lies in the ultralow regime and is smaller than those of both BiOCuSe and Bi₂O₂Se. Bi₄O₄Cu_1.7Se_2.7Cl_0.3 behaves like a charge-balanced semiconductor with a narrow band gap. The chemical diversity offered by the additional anion allows the integration of two common structural units in a single phase by the simultaneous and coupled creation of charge-balancing defects in each of the units