Bibliografía

Reaction of the anion-deficient, cation-ordered perovskite phase Ba₂YFeO₅ with 80 atm of oxygen pressure at 410 °C results in the formation of the Fe⁴⁺ phase Ba₂YFeO_5.5. The topochemical insertion of oxide ions lifts the inversion symmetry of the centrosymmetric host phase, Ba₂YFeO₅ (space group <i>P</i>2₁/<i>n</i>), to yield a noncentrosymmetric (NCS) phase Ba₂YFeO_5.5 (space group <i>Pb</i>2₁<i>m</i> (No. 26), <i>a</i> = 12.1320(2) Å, <i>b</i> = 6.0606(1) Å, <i>c</i> = 8.0956(1) Å, <i>V</i> = 595.257(2) ų) confirmed by the observation of second-harmonic generation. Dielectric and PUND ferroelectric measurements, however, show no evidence for a switchable ferroelectric polarization, limiting the material to pyroelectric behavior. Magnetization and low-temperature neutron diffraction data indicate that Ba₂YFeO_5.5 undergoes a magnetic transition at 20 K to adopt a state which exhibits a combination of ferromagnetic and antiferromagnetic order. The symmetry breaking from centrosymmetric to polar noncentrosymmetric, which occurs during the topochemical oxidation process is discussed on the basis of induced lattice strain and an electronic instability and represents a new strategy for the preparation of NCS materials that readily incorporate paramagnetic transition metal centers

Structure and Magnetism of Sr₃Co₂O₄Cl₂An Electronically Driven Lattice Distortion in an Oxychloride Containing Square Planar Co^II Centers

Topochemical reduction of Sr₃Co₂O₅Cl₂ with NaH at 200 °C yields Sr₃Co₂O₄Cl₂, a phase consisting of infinite double sheets of corner-linked Co^IIO₄ square planes stacked with SrCl rocksalt layers. At 298 K, the structure of Sr₃Co₂O₄Cl₂ is described in the tetragonal space group <i>I</i>4/<i>mmm</i> [<i>a</i> = 4.007(1) Å, <i>c</i> = 22.282(1) Å]; however, on cooling below 200 K, the structure undergoes a lattice distortion to adopt a structure with orthorhombic symmetry in space group <i>Immm</i> [<i>a</i> = 3.9757(5) Å, <i>b</i> = 4.0294(5) Å, and <i>c</i> = 22.147(3) Å at 5 K]. The structural distortion can be considered Jahn–Teller-like as it lifts the orbital degeneracy of the square planar, d⁷, Co^II centers, demonstrating a strong coupling between the electronic configuration and the structural lattice of this oxychloride phase. On cooling below 50 K, Sr₃Co₂O₄Cl₂ adopts a canted antiferromagnetically ordered state. All magnetization data show that the local cobalt moment is much greater than would be expected for a simple spin-only <i>S</i> = ³/₂ center, indicating a strong orbital contribution to the magnetic behavior

Directed Lifting of Inversion Symmetry in Ruddlesden–Popper Oxide–Fluorides: Toward Ferroelectric and Multiferroic Behavior

The cooperative tilting distortions of <i>n</i> = 2 Ruddlesden–Popper oxides can be utilized to break the inversion symmetry of the host lattice and induce ferroelectric behavior. Unfortunately the desired a^–a^–c⁺/a^–a^–c⁺ structural deformation is only stabilized in phases with extremely small structural tolerance factors, limiting the chemical scope of this symmetry breaking approach. Here we describe the influence of topochemical fluorination on the structural distortions of <i>n</i> = 2 Ruddlesden–Popper oxides and demonstrate that the conversion of La₃Ni₂O₇ to La₃Ni₂O_5.5F_3.5 breaks the inversion symmetry of the perovskite double layers which constitute the Ruddlesden–Popper framework, by driving a change from an a^–a^–c⁰/a^–a^–c⁰ distortion in the parent phase to an a^–a^–c⁺/a^–a^–-(c⁺) distortion in the oxide–fluoride. In this instance, the symmetry breaking distortions of adjacent acentric perovskite sheets are antialigned, and as a result the inversion symmetry of the host lattice is broken locally, but not globally, resulting in an antiferroelectric structure. The breaking of local inversion symmetry in layered perovskite phases, in the absence of second-order Jahn–Teller active “distortion centers”, is an important step toward the realization of ferroelectric and multiferroic behavior in phases of this structure type

Diamagnetic Ru²⁺ in Na₂La₂Ti₂RuO_{10–<i>x</i>} (0 < <i>x</i> < 2): A Series of Complex Oxides Prepared by Topochemical Reduction

Reaction of the <i>n</i> = 3 Ruddlesden–Popper phase Na₂La₂Ti₂RuO₁₀ with a 5% H₂/95% N₂ atmosphere between 300 and 900 °C leads to the formation of phases of composition Na₂La₂Ti₂RuO_{10–<i>x</i>} (0 < <i>x</i> < 2) via topochemical reduction. Magnetization data collected from Na₂La₂Ti₂RuO_{10–<i>x</i>} samples in the range 0 < <i>x</i> < 1 show a rapid decline in susceptibility with increasing <i>x</i>, consistent with the conversion of S = 1, Ru⁴⁺ centers at <i>x</i> = 0 to S = 0, Ru²⁺ centers at <i>x</i> = 1. We believe this is the first report of diamagnetic Ru²⁺ centers in an extended oxide phase. Further reduction of Na₂La₂Ti₂RuO₉ leads to the reduction of Ti⁴⁺ to Ti³⁺; however, Na₂La₂Ti₂RuO_{10–<i>x</i>} samples in the range 1 < <i>x</i> < 2 exhibit only a very weak paramagnetic response. Given the highly insulating nature of the phases, this suggests the electrons added on reduction of titanium are paired within a local Ti–Ti bonding network in a manner analogous to that observed for Ti_<i>n</i>O_{2<i>n</i>–1} phases

K₄Fe₃F₁₂: An Fe²⁺/Fe³⁺ Charge-Ordered, Ferrimagnetic Fluoride with a Cation-Deficient, Layered Perovskite Structure

A new mixed-valence iron (Fe²⁺/Fe³⁺) fluoride material with a layered perovskite-related structure has been synthesized and characterized. The material, K₄Fe₃F₁₂ [K₄(Fe²⁺)­(Fe³⁺)₂F₁₂], was synthesized using mild hydrothermal conditions. The material exhibits a layered perovskite structure consisting of alternating sheets of apex-linked Fe²⁺F₆ and Fe³⁺F₆ octahedra; thus, each layer of Fe²⁺F₆ centers is sandwiched between two layers of Fe³⁺F₆ centers. Magnetization and neutron powder diffraction data show that, upon cooling below 120 K, K₄Fe₃F₁₂ adopts a magnetically ordered state in which the Fe³⁺ and Fe²⁺ spins are aligned in an approximately antiparallel manner to each other to yield a pseudoferrimagnetic structure with a net spontaneous moment of 5.41 μ_B per formula unit at 10 K. Crystal data: K₄Fe₃F₁₂, trigonal space group <i>R</i>3̅<i>m</i> (No. 166), <i>a</i> = <i>b</i> = 5.7649(9) Å, <i>c</i> = 28.086(9) Å, <i>V</i> = 808.36(3) ų, <i>Z</i> = 3, <i>T</i> = 296(2) K

Synthesis and Selective Topochemical Fluorination of the Cation and Anion-Vacancy Ordered phases Ba₂YCoO₅ and Ba₃YCo₂O_7.5

The synthesis and characterization of two cation-ordered, anion-vacancy ordered phases, Ba₂YCoO₅ and Ba₃YCo₂O_7.5, is described. Neutron powder diffraction data reveal both phases adopt structures in which octahedral Y³⁺ and tetrahedral Co³⁺ centers are ordered within a “cubic” perovskite lattice. The unusual ordered pattern adopted by the cations can be attributed to the large concentration of anion vacancies within each phase. Reaction of Ba₂YCoO₅ with CuF₂ under flowing oxygen topochemically inserts fluorine into the host material to form Ba₂YCoO₅F_0.42(1). In contrast Ba₂YCoO₅ does not intercalate oxygen, even under high oxygen pressure. The selective insertion of fluorine, but not oxygen, into Ba₂YCoO₅ is discussed and rationalized on the basis of the lattice strain of the resulting oxidized materials. Magnetization and neutron diffraction data reveal Ba₃YCo₂O_7.5 and Ba₂YCoO₅F_0.42 adopt antiferromagnetically ordered states at low-temperature, while in contrast Ba₂YCoO₅ shows no sign of long-range magnetic order

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

Coupled Electronic and Magnetic Phase Transition in the Infinite-Layer Phase LaSrNiRuO₄

Topochemical reduction of the ordered double perovskite LaSrNiRuO₆ with CaH₂ yields LaSrNiRuO₄, an extended oxide phase containing infinite sheets of apex-linked, square-planar Ni¹⁺O₄ and Ru²⁺O₄ units ordered in a checkerboard arrangement. At room temperature the localized Ni¹⁺ (d⁹, <i>S</i> = ¹/₂) and Ru²⁺ (d⁶, <i>S</i> = 1) centers behave paramagnetically. However, on cooling below 250 K the system undergoes a cooperative phase transition in which the nickel spins align ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. Features of the low-temperature crystal structure suggest a symmetry lowering Jahn–Teller distortion could be responsible for the observed diamagnetism of the ruthenium centers

Cation Exchange in a 3D PerovskiteSynthesis of Ni_0.5TaO₃

Reaction of NiCl₂ with NaTaO₃ leads the formation of the perovskite phase Ni_0.5TaO₃, via a topochemical nickel-for-sodium cation exchange in which the framework of apex-linked TaO₆ octahedra present in the parent phase is retained. Neutron powder diffraction data indicate Ni_0.5TaO₃ adopts a structure analogous to the paraelectric phase of LiTaO₃, with triclinic <i>P</i>1̅ crystallographic symmetry. Although Ni_0.5TaO₃ has features which make it a good candidate phase for magnetoelectric multiferroic behavior, the phase remains paramagnetic in the temperature range 15 < <i>T</i> (K) < 300, and detailed crystallographic characterization and analysis of SHG activity indicate it retains a centrosymmetric structure down to the lowest temperatures measured (5 K). Topochemical cation exchange reactions of 3D perovskite oxides offer the opportunity to prepare a wide range of novel metastable phases in a rational manner with a high degree of synthetic control

Structural Modification of the Cation-Ordered Ruddlesden–Popper Phase YSr₂Mn₂O₇ by Cation Exchange and Anion Insertion

Calcium-for-strontium cation substitution of the a^–b⁰c⁰/b⁰a^–c⁰-distorted, cation-ordered, <i>n</i> = 2 Ruddlesden–Popper phase, YSr₂Mn₂O₇, leads to separation into two phases, which both retain an a^–b⁰c⁰/b⁰a^–c⁰-distorted framework and have the same stoichiometry but exhibit different degrees of Y/Sr/Ca cation order. Increasing the calcium concentration to form YSr_0.5Ca_1.5Mn₂O₇ leads to a change in the cooperative tilting on the MnO₆ units to a novel a^–b^–c^–/b^–a^–c^– arrangement described in space group <i>P</i>2₁/<i>n</i>11. Low-temperature, topochemical fluorination of YSr₂Mn₂O₇ yields YSr₂Mn₂O_5.5F_3.5. In contrast to many other fluorinated <i>n</i> = 2 Ruddlesden–Popper oxide phases, YSr₂Mn₂O_5.5F_3.5 retains the a^–b⁰c⁰/b⁰a^–c⁰ lattice distortion and <i>P</i>4₂/<i>mnm</i> space group symmetry of the parent oxide phase. The resilience of the a^–b⁰c⁰/b⁰a^–c⁰-distorted framework of YSr₂Mn₂O₇ to resist symmetry-changing deformations upon both cation substitution and anion insertion/exchange is discussed on the basis the A-site cation order of the lattice and the large change in the ionic radius of manganese upon oxidation from Mn³⁺ to Mn⁴⁺. The structure property relations observed in the Y–Sr–Ca–Mn–O–F system provide insight into assisting in the synthesis of <i>n</i> = 2 Ruddlesden–Popper phases, which adopt cooperative structural distortions that break the inversion symmetry of the extended lattice and therefore act as a route for the preparation of ferroelectric and multiferroic materials