Sr2FeO3 with Stacked Infinite Chains of FeO4 Square Planes

Interplay between A-site and oxygen-vacancy ordering, and mixed electron/oxide-ion conductivity in n = 1 Ruddlesden-Popper perovskite Sr2Nd2Zn2O7

Oxygen vacancies in Ruddlesden-Popper (RP) perovskites (PV) [AO][ABO3] n play a pivotal role in engineering functional properties and thus understanding the relationship between oxygen-vacancy distribution and physical properties can open up new strategies for fine manipulation of structure-driven functionalities. However, the structural origin of preferential distribution for oxygen vacancies in RP structures is not well understood, notably in the single-layer (n = 1) RP-structure. Herein, the n = 1 RP phase Sr2Nd2Zn2O7 was rationally designed and structurally characterized by combining three-dimensional (3D) electron diffraction and neutron powder diffraction. Sr2Nd2Zn2O7 adopts a novel 2-fold n = 1 RP-type Pmmn-superstructure due to the concurrence of A-site column ordering and oxygen-vacancy array ordering. These two ordering models are inextricably linked, and disrupting one would thus destroy the other. Oxygen vacancies are structurally confined to occupy the equatorial sites of "BO6"-octahedra, in stark contrast to the preferential occupation of the inner apical sites in n ≥ 2 structures. Such a layer-dependent oxygen-vacancy distribution in RP structures is in fact dictated by the reduction of the cationic A-A/B repulsion. Moreover, the intrinsic oxygen vacancies can capture atmospheric O2, consequently resulting in a mixed oxide ion and p-type electrical conductivity of 1.0 × 10-4 S cm-1 at temperatures > 800 °C. This value could be further enhanced to > 1.0 × 10-3 S cm-1 by creating additional oxygen vacancies on the equatorial sites through acceptor doping. Bond valence site energy analysis indicates that the oxide ion conduction in Sr2Nd2Zn2O7 is predominated by the one-dimensional pathways along the [Zn2O7] ladders and is triggered by the gate-control-like migration of the equatorial bridging oxygens to the oxygen-vacant sites. Our results demonstrate that control of anion and cation ordering in RP perovskites opens a new path toward innovative structure-driven property design.

Tuning Optical and Electrical Properties of Vanadium Oxide with Topochemical Reduction and Substitutional Tin

Vanadium oxides are widely tunable materials, with many thermodynamically stable phases suitable for applications spanning catalysis to neuromorphic computing. The stability of vanadium in a range of oxidation states enables mixed-valence polymorphs of kinetically accessible metastable materials. Low-temperature synthetic routes to, and the properties of, these metastable materials are poorly understood and may unlock new optoelectronic and magnetic functionalities for expanded applications. In this work, we demonstrate topochemical reduction of α-V2O5 to produce metastable vanadium oxide phases with tunable oxygen vacancies (>6%) and simultaneous substitutional tin incorporation (>3.5%). The chemistry is carried out at low temperature (65 °C) with solution-phase SnCl2, where Sn2+ is oxidized to Sn4+ as V5+ sites are reduced to V4+ during oxygen vacancy formation. Despite high oxygen vacancy and tin concentrations, the transformations are topochemical in that the symmetry of the parent crystal remains intact, although the unit cell expands. Band structure calculations show that these vacancies contribute electrons to the lattice, whereas substitutional tin contributes holes, yielding a compensation doping effect and control over the electronic properties. The SnCl2 redox chemistry is effective on both solution-processed V2O5 nanoparticle inks and mesoporous films cast from untreated inks, enabling versatile routes toward functional films with tunable optical and electronic properties. The electrical conductance rises concomitantly with the SnCl2 concentration and treatment time, indicating a net increase in density of free electrons in the host lattice. This work provides a valuable demonstration of kinetic tailoring of electronic properties of vanadium-oxygen systems through top-down chemical manipulation from known thermodynamic phases.

Band engineering of layered oxyhalide photocatalysts for visible-light water splitting

The band structure offers fundamental information on electronic properties of solid state materials, and hence it is crucial for solid state chemists to understand and predict the relationship between the band structure and electronic structure to design chemical and physical properties. Here, we review layered oxyhalide photocatalysts for water splitting with a particular emphasis on band structure control. The unique feature of these materials including Sillén and Sillén-Aurivillius oxyhalides lies in their band structure including a remarkably high oxygen band, allowing them to exhibit both visible light responsiveness and photocatalytic stability unlike conventional mixed anion compounds, which show good light absorption, but frequently encounter stability issues. For band structure control, simple strategies effective in mixed-anion compounds, such as anion substitution forming high energy p orbitals in accordance with its electronegativity, is not effective for oxyhalides with high oxygen bands. We overview key concepts for band structure control of oxyhalide photocatalysts such as lone-pair interactions and electrostatic interactions. The control of the band structure of inorganic solid materials is a crucial challenge across a wide range of materials chemistry fields, and the insights obtained by the development of oxyhalide photocatalysts are expected to provide knowledge for diverse materials chemistry.

Peroxymonosulfate activation using heterogeneous catalyst Sr2FeO4 coated on SBA-15 for efficient degradation of antibiotic sulfapyridine

It is significant to explore the advanced oxidation process (AOP) for antibiotic degradation. Herein, a peroxymonosulfate (PMS) activator, Sr₂FeO₄/SBA-15 (SFS) heterogeneous catalyst, was synthesized by in situ growth of Sr₂FeO₄ on the surface of SBA-15. In SFS/PMS catalytic system, Sr atom provided electrons to Fe(II) ↔Fe(III) ↔Fe(II) redox cycle through Sr-O-Fe bonds for PMS activation. The SFS catalyst could activate PMS to generate a free radical coexistence system, including sulfate radical (SO₄∙^-) and hydroxyl radicals (∙OH). The catalyst possessed high catalytic activity and high stability. The degradation efficiency of sulfapyridine (SAD) over the SFS/PMS catalytic system could reach 99.0% after 90 min reaction. After the 5th reuse, the degradation efficiency of SAD was still more than 94.0%, and the phase structure of the catalyst did not alter. The low ion leaching concentration would be more conducive to reuse and avoiding secondary pollution, in comparison to homogeneous catalysts. This catalyst can be widely applied to organic wastewater treatment.-->.

Oxygen deficiency in Sr2FeO4-x: electrochemical control and impact on magnetic properties

The oxygen-deficient system Sr₂FeO_4-x was explored by heating the stoichiometric Fe⁴⁺ oxide Sr₂FeO₄ in well-defined oxygen partial pressures which were controlled electrochemically by solid-state electrolyte coulometry. Samples with x up to about 0.2 were obtained by this route. X-ray diffraction analysis reveals that the K₂NiF₄-type crystal structure (space group I4/mmm) of the parent compound is retained. The lattice parameter a slightly decreases while the c-parameter increases with increasing x, which is in contrast to the Ruddlesden-Popper system Sr₃Fe₂O_7-x and suggests removal of oxygen atoms from FeO₂ lattice planes. The magnetic properties were studied by magnetization, ⁵⁷Fe Mössbauer, and powder neutron diffraction experiments. The results suggest that extraction of oxygen atoms from the lattice progressively changes the elliptical spiral spin ordering of the parent compound to an inhomogeneous magnetic state with coexistence of long-range ordered regions adopting a circular spin spiral and smaller magnetic clusters.

Hole and Electron Doping of Topochemically Reduced Ni(I)/Ru(II) Insulating Ferromagnetic Oxides

La_xSr_2-xNiRuO₆, La_xSr_4-xNiRuO₈, and La_xSr_3-xNiRuO₇ are, respectively, the n = ∞, 1, and 2 members of the (La_x/2Sr_1-(x/2))_nSr(Ni_0.5Ru_0.5)_nO_3n+1 compositional series. Reaction with CaH₂, in the case of the La_xSr_2-xNiRuO₆ perovskite phases, or Zr oxygen getters in the case of the La_xSr_4-xNiRuO₈ and La_xSr_3-xNiRuO₇ Ruddlesden-Popper phases, yields the corresponding topochemically reduced (La_x/2Sr_1-(x/2))_nSr(Ni_0.5Ru_0.5)_nO_3n-1 compounds (La_xSr_2-xNiRuO₄, La_xSr_4-xNiRuO₆, and La_xSr_3-xNiRuO₅), which contain Ni and Ru cations in square-planar coordination sites. The x = 1 members of each series (LaSrNiRuO₄, LaSr₃NiRuO₆, and LaSr₂NiRuO₅) exhibit insulating ferromagnetic behavior at low temperature, attributable to exchange couplings between the Ni¹⁺ and Ru²⁺ centers they contain. Increasing the La³⁺ concentration (x > 1) leads to a reduction of some of the Ru²⁺ centers to Ru¹⁺ centers and a suppression of the ferromagnetic state (lower T_c, reduced saturated ferromagnet moment). In contrast, increasing the Sr²⁺ concentration (x < 1) oxidizes some of the Ru²⁺ centers to Ru³⁺ centers and enhances the ferromagnetic coupling (increased T_c, increased saturated ferromagnet moment) for the n = ∞ and n = 2 samples but appears to have no influence on the magnetic ordering temperature of the n = 1 samples. The magnetic couplings and influence of doping are discussed on the basis of superexchange and direct exchange couplings between the square-planar Ni and Ru centers.

Hydride-Reduced Eu2SrFe2O6: A T-to-T' Conversion Enabling Fe2+ in Square-Planar Coordination

Low-temperature reaction of A-site-ordered layered perovskite Eu₂SrFe₂O₇ (T structure) with CaH₂ induces a shift in the Eu₂O₂ slabs to form Eu₂SrFe₂O₆ with a T' structure (I4/mmm space group) in which only the Fe cation is reduced. Contrary to the previously reported T' structures with Jahn-Teller-active d⁹ cations (Cu²⁺ and Ni⁺), stabilization of Eu₂SrFe₂O₆ with the Fe²⁺ (d⁶) cation reflects the stability of the FeO₄ square-planar unit. The stability of T'-type Eu₂SrFe₂O₆ over a T-type polymorph is confirmed by density functional theory calculations, revealing the d_z² occupancy for the T' structure. Eu₂SrFe₂O₆ has a bilayer magnetic framework with an Fe-O-Fe superexchange J_∥ and an Fe-Fe direct exchange J_⊥ (where J_∥ > J_⊥), which broadly explains the observed T_N of 390-404 K. Interestingly, the magnetic moments of Eu₂SrFe₂O₆ lie in the ab plane, in contrast to the structurally similar Sr₃Fe₂O₄Cl₂ having an out-of-plane spin alignment.

Electron Doping BaZrO3 via Topochemical Reduction

We report the topochemical reduction of epitaxial thin films of the cubic perovskite BaZrO₃. Reduction with calcium hydride yields n-type conductivity in the films, despite the wide band gap and low electron affinity of the parent material. X-ray diffraction studies show concurrent loss of out-of-plane texture with stronger reducing conditions. Temperature-dependent transport studies on reduced films show insulating behavior (decreasing resistivity with increasing temperature) with a combination of thermally activated and variable-range hopping transport mechanisms. Time-dependent conductivity studies show that the films are stable over short periods, with chemical changes over the course of weeks leading to an increase in electrical resistance. Neutron reflectivity and secondary ion mass spectrometry indicate that the source of the carriers is most likely hydrogen incorporated from the reducing agent occupying oxygen vacancies and/or interstitial sites. Our studies introduce topochemical reduction as a viable pathway to electron-dope and meta-stabilize low electron affinity and work function materials.

Redetermination of Sr2PdO3 from single-crystal X-ray data

The crystal structure redetermination of Sr2PdO3 (distrontium palladium trioxide) was carried out using high-quality single-crystal X-ray data. The Sr2PdO3 structure has been described previously in at least three reports [Wasel-Nielen & Hoppe (1970 ▸). Z. Anorg. Allg. Chem. 375, 209-213; Muller & Roy (1971 ▸). Adv. Chem. Ser. 98, 28-38; Nagata et al. (2002 ▸). J. Alloys Compd. 346, 50-56], all based on powder X-ray diffraction data. The current structure refinement of Sr2PdO3, as compared to previous powder data refinements, leads to more precise cell parameters and fractional coordinates, together with anisotropic displacement parameters for all sites. The compound is confirmed to have the ortho-rhom-bic Sr2CuO3 structure type (space group Immm) as reported previously. The structure consists of infinite chains of corner-sharing PdO4 plaquettes inter-spersed by SrII atoms. A brief comparison of Sr2PdO3 with the related K2NiF4 structure type is given.

Soft chemistry of ion-exchangeable layered metal oxides

Disassembly and re-assembly of layered metal oxides by soft chemical approaches can be used to tailor functionalities in artificial photosynthesis, energy storage, optics, and piezoelectrics.

Pressure induced structural and spin state transitions in Sr3Fe2O5

The effect of pressure on the structural, electronic and magnetic properties of the two-legged spin ladder structure Sr₃Fe₂O₅ was investigated, using density functional theory within the generalized gradient approximation (GGA) + U method.

Strontium vanadium oxide-hydrides: "square-planar" two-electron phases

A series of strontium vanadium oxide-hydride phases prepared by utilizing a low-temperature synthesis strategy in which oxide ions in Sr(n+1)V(n)O(3n+1) (n=∞, 1, 2) phases are topochemically replaced by hydride ions to form SrVO2H, Sr2VO3H, and Sr3V2O5H2, respectively. These new phases contain sheets or chains of apex-linked V(3+)O4 squares stacked with SrH layers/chains, such that the n=∞ member, SrVO2H, can be considered to be analogous to "infinite-layer" phases, such as Sr(1-x)Ca(x)CuO2 (the parent phase of the high-T(c) cuprate superconductors), but with a d(2) electron count. All three oxide-hydride phases exhibit strong antiferromagnetic coupling, with SrVO2H exhibiting an antiferromagnetic ordering temperature, T(N)>300 K. The strong antiferromagnetic couplings are surprising given they appear to arise from π-type magnetic exchange.