Photocatalytic Water Oxidation by a Pyrochlore Oxide upon Irradiation with Visible Light: Rhodium Substitution Into Yttrium Titanate

A stable visible‐light‐driven photocatalyst (λ≥450 nm) for water oxidation is reported. Rhodium substitution into the pyrochlore Y₂Ti₂O₇ is demonstrated by monitoring Vegard′s law evolution of the unit‐cell parameters with changing rhodium content, to a maximum content of 3 % dopant. Substitution renders the solid solutions visible‐light active. The overall rate of oxygen evolution is comparable to WO₃ but with superior light‐harvesting and surface‐area‐normalized turnover rates, making Y₂Ti_1.94Rh_0.06O₇ an excellent candidate for use in a Z‐scheme water‐splitting system.     

A 3D Porous Metal Organic Framework Based on Infinite 1D Nickel(II) Chains with Rutile Topology Displaying Open Metal Sites

The reaction of Ni^II with a tetra‐benzoate pyrene ligand produces a 3D porous framework based on infinite 1D Ni^II chains. The Ni^II–O connectivity and the formation of a hydroxo‐bridge (μ₃‐OH) responsible for the connection of the central Ni^II atoms within the 1D Ni^II–(μ₃‐OH)₂–Ni^II chains can be straightforwardly compared with the Ti^IV–O–Ti^IV connectivity seen in TiO₂. The arrangement of the TBAPy ligand around the 1D rutile‐based chains leads in the generation of a porous framework with two distinct types of pores; based on the chemistries of these two types of pores, one can be labelled as hydrophobic and the other as hydrophilic. The use of different activation methods results in the generation of either a porous framework free of guest molecules or a completely solvent‐free material, in which the terminal H₂O molecules bound to Ni^II were removed, leading thus to a framework with open Ni^II sites. CO₂ isotherms collected on both frameworks at 195 K and one barshowed type I isotherms characteristic of microporous materials (BET surface areas for: guest‐free framework: 257(3) m² · g^–1; solvent‐free framework: 362(2) m² · g^–1). The affinity of both networks at zero coverage for both CO₂ and CH₄ was found to be greater when the unsaturated Ni^II sites are available within the void space.     

Probing magnetic order in heavily doped La2-xSrxNiO4+\delta with μSR

We report μSR measurements on a series of compounds with composition La_2-xSr_xNiO_4+\delta where the net hole concentration x+2\delta is greater than 0.4. A magnetic transition is found in all the samples studied which occurs at a composition‐dependent temperature T_M. Below T_M, clear precession signals are observed in zero applied magnetic field. The possible muon sites are discussed within the context of dipole‐field calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Ca(2.5)Sr(0.5)GaMn2O8: diamagnetic Ga in control of the structural and electronic properties of a bilayered manganate

The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Three-coordinate metal centers in extended transition metal oxides

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure.     

Crystal Structure and Magnetic Properties of SrCaMnGaO5+δ

The room-temperature crystal structure of the brownmillerite SrCaMnGaO_5+δ (δ=0.035) has been refined from neutron powder diffraction data; space group Ima2, a=15.7817(6), b=5.4925(2), c=5.3196(2)> Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μ_B per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6 T, increasing to 90% in 13 T.     

Close-packed C(70)(3-) phases - synthesis, structure, and electronic properties

The high symmetry and resulting electronic degeneracy of the C(60)(3)(-) anion is viewed as the key molecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerene systems. The experimental evaluation of this hypothesis requires the synthesis of face-centered cubic (fcc) trivalent fulleride anion salts derived from higher fullerenes such as C(70), which have thus far proved elusive with only stable A(1)C(70), A(4)C(70), and A(6)C(70) phases known. In this paper, we report the synthesis of fcc A(3)C(70) phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are strongly dependent on the cooling protocol due to the existence of metastable partially or completely orientationally disordered phases. EPR data indicate that the phases are metallic but not superconducting. The densities of states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recent observations that four electrons per C(70) anion are required for superconductivity. Size-matching on both the octahedral and tetrahedral sites is required for A(3)C(70) stability - K(2)CsC(70) is only stable at elevated temperature and Na(2)C(70) is unstable, the composition corresponding to C(70) and a sodium-rich trigonal phase.