Synthesis and characterization of the reduced double-layer manganite Sr3Mn2O6+x

The oxygen-deficient Ruddlesden-Popper (RP) phase Sr₃Mn₂O₆ crystallizes with an ordered array of oxygen vacancies to afford a structure in which the Mn³⁺ ions exist in a square-pyramidal environment. The MnO₅ polyhedra are linked through their corners to form a structure that is related to that observed for the single-layered material, Sr₂MnO_3.5. The nuclear and magnetic structures of a polycrystalline sample of Sr₃Mn₂O₆ have been determined using Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn³⁺ double-layered phase crystallizes in a superstructure of the simple RP subcell: tetragonal, P4/mbm, a=10.8686(2) Å and c=20.2051(3) Å.Magnetic susceptibility studies suggest a transition at ∼250 K to a canted antiferromagnetic ordered structure. The magnetic unit-cell consists of ferromagnetic clusters of corner-sharing MnO₅ units, which are antiferromagnetically aligned to other clusters within the layers.     

SrMn3O6: an incommensurate modulated tunnel structure

The crystallographic structure of a mixed valent manganite SrMn₃O₆ with a 1D modulated structure is reported. The SrMn₃O₆ structure can be described with the basic subcell space group Pnma (a=9.1334(5) Å; b=2.8219(2) Å; c=12.0959(7)Å), but transmission electron microscopy revealed that the study of the real structure requires a 4D-formalism approach with superspace group P2₁/a(αβ0)00 (unique axis c), with a modulation wave vector q having the approximate components (0.52a*+0.31b*). The crystallographic structure is closely related to those of Na_xFe_xTi_2−xO₄ and Pb_1.5BaMn₆Al₂O₁₆, comprising of unusual “figure-of-eight”-shaped tunnels, made up of strings of edge- and corner-sharing (Fe/Ti)O₆ or (Mn/Al)O₆ octahedra, with the other cations situated in the tunnel cavities. Structural refinement was performed on X-ray and neutron powder diffraction data using the 4D formalism. All atoms in the crystal are affected by a displacive modulation wave, and a saw tooth function is employed to model the displacement and occupancy of the Sr sites. Magnetic susceptibility measurements reveal a sharp antiferromagnetic transition with T_N∼46 K.     

Non‐Newtonian Polymer–Nanoparticle Hydrogels Enhance Cell Viability during Injection

Drug delivery and cell transplantation require minimally invasive deployment strategies such as injection through clinically relevant high‐gauge needles. Supramolecular hydrogels comprising dodecyl‐modified hydroxypropylmethylcellulose and poly(ethylene glycol)‐block‐poly(lactic acid) have been previously demonstrated for the delivery of drugs and proteins. Here, it is demonstrated that the rheological properties of these hydrogels allow for facile injectability, an increase of cell viability after injection when compared to cell viabilities of cells injected in phosphate‐buffered saline, and homogeneous cell suspensions that do not settle. These hydrogels are injected at 1 mL min^?1 with pressures less than 400 kPa, despite the solid‐like properties of the gel when at rest. The cell viabilities immediately after injection are greater than 86% for adult human dermal fibroblasts, human umbilical vein cells, smooth muscle cells, and human mesenchymal stem cells. Cells are shown to remain suspended and proliferate in the hydrogel at the same rate as observed in cell media. The work expands on the versatility of these hydrogels and lays a foundation for the codelivery of drugs, proteins, and cells.     

Crystallographic and magnetic structures of Y0.8Sr2.2Mn2GaO8−δ: a new vacancy-ordered perovskite structure

The anion-deficient perovskite Y_0.8Sr_2.2Mn₂GaO_8−δ (where δ∼0.1) has been synthesised and the crystal and magnetic structures determined by Rietveld analysis of neutron powder diffraction (NPD) data. The material has body-centred tetragonal symmetry (I4/mmm, a=7.6373(3) Å and c=15.6636(10) Å) and consists of alternating layers of octahedral and tetrahedral polyhedra, the layers being perpendicular to [001]. The octahedral layers are preferentially occupied by manganese and the tetrahedral layers are a mixture of manganese and gallium. The precise cation distribution depends critically on preparative conditions. An unusual structural feature of these materials is the arrangement of oxygen vacancies in the tetrahedral layers: in the basic structure, isolated squares of corner-linked tetrahedra are formed instead of the chains that are observed in brownmillerite phases. Additional oxide ions in this layer probably allow the Mn ions to achieve distorted square pyramidal coordination. Low temperature NPD and magnetisation data indicate antiferromagnetic ordering below 100 K.     

Coagulation of poly(p-phenylene benzobisoxazole) in polyphosphoric acid using aqueous and nonaqueous solutions

Diffusion of aqueous and nonaqueous coagulants into lyotropic/nematic solutions of poly(phenylene benzobisoxazole) in polyphosphoric acid using fluorescence microscopy is reported. The fluorescence microscopy technique offers advantages in that the experiment is independent of the coagulant and the experiments can be performed in a front surface mode so that optically thin samples are not required. The diffusion process is modeled as Case II diffusion as described by Crank and Park. The apparent diffusion coefficients are found to range from ～ 2 × 10^?6 cm²/s to ～ 2 × 10^?7 cm²/s for aqueous and nonaqueous coagulants. The apparent diffusion coefficients can be varied by several orders of magnitude (e.g., down to ～ 3 × 10^?10 cm²/s) by varying the concentration of polyphosphoric acid in the coagulant. © 1995 John Wiley & Sons, Inc.     

Synthesis, structural characterization and Li ion conductivity of a new vanado-molybdate phase, LiMg3VMo2O12

A new vanado-molybdate LiMg₃VMo₂O₁₂ has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li_2−zMg_2+zV_zMo_3−zO₁₂ was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li₂M₂(MoO₄)₃ (M=M²⁺, z=0). The structures comprise of two unique (Li/Mg)O₆ octahedra, (Li/Mg)O₆ trigonal prisms and two unique (Mo/V)O₄ tetrahedra. A well-defined 1:3 ratio of Li⁺:Mg²⁺ is observed in octahedral chains for LiMg₃VMo₂O₁₂. Li⁺ preferentially occupies trigonal prisms and Mg²⁺ favours octahedral sheets. Excess V⁵⁺ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li⁺ ion conductivity occurs at the phase transition and the high temperature structure is a good Li⁺ ion conductor, with σ=1×10⁻³-4×10⁻² S cm⁻¹ and E_a=0.6 to 0.8 eV.     

Plasma-promoted dielectric heating in the microwave synthesis of spinels

Comparison between the rates of synthesis of the spinel phases (MAl2O4, M = Mg, Zn, Ni; and MFe2O4, M = Mg, Zn, Mn) using an O2 microwave-induced plasma (MIP) and conventional (tube furnace) methods indicate that a MIP promotes microwave dielectric heating at elevated temperatures.     

Position measurements in the de Broglie–Bohm interpretation of quantum mechanics

The de Broglie–Bohm interpretation of quantum mechanics assigns positions and trajectories to particles. We analyze the validity of a formula for the velocities of Bohmian particles which makes the analysis of these trajectories particularly simple. We apply it to particle detectors of four different types and show that the detectors of three of these types lead to “surrealistic trajectories”, i.e., leave a trace where the Bohmian particle was not present.