Sixfold Oxygen Coordination Research Articles

Determination of Mg(2+) Speciation in a TFSI(-)-Based Ionic Liquid With and Without Chelating Ethers Using Raman Spectroscopy.

Raman spectroscopy was employed to assess the complex environment of magnesium salts in the n-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrTFSI) room-temperature ionic liquid (RTIL). At room temperature, Mg(TFSI)2 was miscible with BMPyrTFSI and formulated by [Mg(TFSI)2](x)[BMPyrTFSI](1-x) (x ≤ 0.55). Results suggest that at low concentrations of Mg(TFSI)2, anionic complexes in which Mg(2+) is surrounded by at least four TFSI(-) were formed. Above x = 0.2 an average of three TFSI(-) surround each Mg(2+). Below x = 0.12, there is a greater number of monodentate interactions between TFSI(-) oxygens and Mg(2+) cations, whereas above x = 0.12 bidentate ligands dominate. The fraction of TFSI(-) existing in the cis conformation increased with increasing Mg(2+) concentration. Mg(ClO4)2 was also studied as a Mg(2+) source. At equivalent mole fractions to those of the Mg(TFSI)2 salt, Mg(2+) from Mg(ClO4)2 was surrounded by only two TFSI(-) anions as ClO4(-) appeared to compete with TFSI(-) for coordination with Mg(2+). Similar behavior was also observed for the less soluble halide salts MgX2 (X = Cl, Br, I). Additions of chelating ligands were shown to effectively reduce the average number of TFSI(-) around Mg(2+) in a manner consistent with maintaining a sixfold oxygen coordination number around Mg(2+). Furthermore, an alternative class of ionic liquids, known as "solvate" ionic liquids, were produced. In this case glymes (Gm, m + 1 ether oxygens) were mixed with Mg(TFSI)2 so that glymes chelated Mg(2+), creating Mg(Gm)(y)(2+) complexes. The general formula was given by Mg(Gm)(y)(TFSI)2. These solvate ILs melt between 40 and 80 °C. Raman spectra clearly showed the glyme chelating ability and stronger coordination with Mg(2+) with respect to TFSI(-). Finally, linear sweep voltammograms showed the anodic stability of the glymes to improve due to coordination with Mg(2+).

Site-specific mapping of transition metal oxygen coordination in complex oxides

We demonstrate site-specific mapping of the oxygen coordination number for transition metals in complex oxides using atomically resolved electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Pb2Sr2Bi2Fe6O16 contains iron with a constant Fe3+ valency in both octahedral and tetragonal pyramidal coordination and is selected to demonstrate the principle of site-specific coordination mapping. Analysis of the site-specific Fe-L2,3 data reveals distinct variations in the fine structure that are attributed to Fe in a six-fold (octahedron) or five-fold (distorted tetragonal pyramid) oxygen coordination. Using these variations, atomic resolution coordination maps are generated that are in excellent agreement with simulations.

ChemInform Abstract: Lanthanum L3-, Europium L3- and Copper K-Edge EXAFS Investigation of Lanthanum Copper Oxides.

Some cation-substituted and oxygen-rich compounds derived from the lanthanum copper oxide of composition La2CuO4 have been examined by measurement of the copper K-, lanthanum L3- and europium L3-edge extended X-ray absorption fine structure. The results obtained from La2CuO4 are in good agreement with previously reported data and are consistent with the six-fold oxygen co-ordination of copper and nine-fold coordination by oxygen of lanthanum. The results obtained from the compound Eu2CuO4 show co-ordination numbers of four and eight for copper and europium as would be expected for a structure containing copper in a square-planar oxygen environment and europium in square antiprismatic oxygen co-ordination.The compound La2CaCu2O6 has a structure in which copper adopts a square-pyramidal oxygen environment whilst the lanthanum atoms are found to be in nine-fold oxygen co-ordination. The oxygen co-ordination of lanthanum and copper in the high-temperature superconductors La1.85Sr0.15CuO4 and La2CuO4.13 is similar to that in La2CuO4. The results obtained from La2CuO4.4 formed by treatment of La2CuO3.6 in oxygen in a thermal analyser are consistent with the formation of an Eu2CuO4-type structure.

Experimental evidence of six-fold oxygen coordination for phosphorus and XANES calculations

Phosphorus, a group V element, has always been found so far in minerals, biological systems and synthetic compounds with an oxygen coordination number of four (i.e, PO4 groups). We demonstrate here using phosphorus K-edge XANES spectroscopy that this element can also adopt a six-fold oxygen coordination (i.e, PO6 groups). This new coordination was achieved in AlPO4 doped SiO2 stishovite synthesized at 18 GPa and 1873 K and quenched down to ambient conditions. The well-crystallized P-bearing stishovite grains (up to 100μm diameter) were embedded in the back-transformation products of high pressure form of AlPO4 matrix. They were identified by elemental mapping (μ-XRF). μ-XANES spectra collected at the Si and P K edges in the Si rich region with a very low concentration of P present striking resemblance, Si itself being characteristic of pure stishovite. We can therefore infer that phosphorus in the corresponding stishovite crystal is involved in an octahedral coordination made of six oxygen atoms. First principle XANES calculations using a plane-wave density functional formalism with core-hole effects treated in a supercell approach at the P K edge for a P atom substituting an Si one in the stishovite structure confirm this assertion. This result shows that in the lower-mantle where all silicon is six-fold coordinated, phosphorus has the crystal-chemical ability to remain incorporated into silicate structures.

Experimental evidence of sixfold oxygen coordination for phosphorus

Phosphorus, a group V element, has always been found so far in minerals, biological systems, and synthetic compounds with an oxygen coordination number of four (i.e., PO 4 groups). We demonstrate using phosphorus K -edge XANES spectroscopy that this element can also adopt a sixfold oxygen coordination (i.e., PO 6 groups). This new coordination was achieved in phosphorus-doped (1 wt% P 2 O 5 level) SiO 2 -stishovite synthesized at 18 GPa and 1873 K and quenched to ambient conditions. This change of phosphorus coordination at high pressure within a dense silicate structure is particularly relevant to phosphorus mineralogy (and geochemistry) in the deep Earth. With a mantle abundance below 0.25 wt%, phosphorus has been shown to be mainly hosted by silicates (e.g., olivine) in the Earth’s upper-mantle, in the fourfold-coordinated silicon sites. In the lower mantle where all silicon is sixfold coordinated, we show here that phosphorus has the crystal-chemical ability to remain incorporated into silicate structures.

Structural characterization of ZnO/ Er2O3 core/shell nanowires

Abstract Zinc oxide/erbium oxide core/shell nanowires are of great potential value to optoelectronics because of the possible demonstration of laser emission in the 1.5 μm range. In this paper we present a convenient technique to obtain structures of this composition. ZnO core nanowires were first obtained by a vapor–liquid–solid (VLS) method using gold as a catalyst. ZnO nanowires ranging from 50 to 100 nm in width were grown on the substrates. Erbium was incorporated into these nanowires by their exposure to Er(tmhd) 3 at elevated temperatures. After annealing at 700 ∘ C in air, the nanowires presented 1.54 μm emission when excited by any of the lines of an Ar + laser. An investigation of nanowire structure by HRTEM indicates that indeed the cores consist of hexagonal ZnO grown in the 001 direction while the surface contains randomly oriented Er 2 O 3 nanoparticles. EXAFS analysis reveals that the Er atoms possess a sixfold oxygen coordination environment, almost identical to that of Er 2 O 3 . Taken collectively, these data suggest that the overall architectures of these nanowires are discrete layered ZnO/ Er 2 O 3 core/shell structures whereby erbium atoms are not incorporated into the ZnO core geometry.

Superconductivity and Mössbauer effect ofFexCu1−xBa2YCu2O7+ysuperconductors synthesized by high pressure

The superconductivity in ${\mathrm{Fe}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{2}\mathrm{Y}{\mathrm{Cu}}_{2}{\mathrm{O}}_{7+y}$ superconductors $(0.00\ensuremath{\leqslant}x\ensuremath{\leqslant}0.70)$ is improved by high-pressure (HP) synthesis. The HP samples have higher oxygen content than the ambient pressure (AM) samples. M\"ossbauer effect studies confirmed that the high-pressure synthesis changes the local structure on the atomic scale. For the HP samples Fe (2) in the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes has highly symmetric pyramidal-oxygen coordination. The tetragonal coordinated Fe (1) in the $\mathrm{Cu}{\mathrm{O}}_{x}$ chains disappears and becomes the one having fivefold and sixfold oxygen coordination after the high-pressure synthesis. This suggests that the increase of oxygen content and the change of local structure are important reasons for the improvement of superconductivity in the HP samples.

Photoluminescence in amorphous zirconium titanate

Photoluminescence at room temperature was observed in amorphous zirconium titanate obtained from the polymeric precursor method. This was the first time in which PL was noticed in an amorphous compound made of two network formers. The PL spectra could be deconvoluted into two bands, whose center 1 was located from 540 nm to 552 nm and center 2 from 625 nm to 641 nm. The co-existence of fivefold and sixfold oxygen coordination of titanium and/or zirconium could be the origin of the radiative recombination of electron-hole pairs in amorphous ZT, which may be responsible for the emission of the photoluminescence.

Dense medium plasma synthesis of carbon/iron-based magnetic nanoparticle system

Using dense medium plasma technology, hybrid iron and iron oxide/carbon-based nanoparticle composites were synthesized under room temperature and atmospheric pressure conditions. Based on results from electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, Raman microscopy, atomic force microscopy and scanning electron microscopy, we conclude that the material is composed of spherical particles, 40–60 nm in diameter, which are a graphitic carbon host structure embedded with iron and iron oxide. Thermal gravimetry/differential thermal gravimetry analysis indicates that these composites are stable up to temperatures as high as 600 °C. Ferromagnetic resonance spectroscopy (FMR) and extended x-ray absorption fine-structure spectroscopy suggest that the bulk of the FMR signal in question is due to metallic Fe. Magnetite or maghemite is present, and the metallic content of the metal particles is 58(8)% and the remainder is oxidized with a sixfold oxygen coordination shell similar to that of γ-Fe2O3.

Ab initio structure determination of In2H2(P2O7)(P4O12) from X-ray powder diffraction data

Abstract The crystal structure of In2H2(P2O7)(P4O12) has been determined from X-ray powder diffraction data. The structure is orthorhombic, space group Pmmn (No. 59), with unit cell dimensions a = 12.9398(2) Å, b = 11.3725(3) Å, c = 5.0385(1) Å, V = 741.45(3) Å3, Z = 2, and D x = 3.23 g/cm3. The structure has been solved by direct methods using EXPO program and refined by FULLPROF package. The final R Bragg value was 4.6%. Positions of hydrogen atoms were not defined. Indium cation has sixfold oxygen coordination being surrounded by two diphosphate and four cyclotetraphosphate groups. Probable scheme of hydrogen bonding is discussed.

The crystal structure of Na3HP2O7 ⋅ H2O from X-ray powder diffraction data

Abstract The crystal structure of Na3HP2O7 ⋅ H2O has been determined from X-ray powder diffraction data. The structure is monoclinic, space group P21/n (No. 14), with unit cell dimensions a = 10.4242(1) Å, b = 6.8707(1) Å, c = 10.0837(1) Å, β = 99.2258(7)°, V = 712.87(2) Å3, Z = 4, D m = 2.43 g/cm3 and D x = 2.41 g/cm3. The structure has been solved by direct methods using EXPO program and refined by FULLPROF package. The final R Bragg value was 7.6%. Positions of hydrogen atoms were not defined. There are three sodium cations in the asymmetric unit. Two of them are five-coordinated and one cation has sixfold oxygen coordination. Each HP2O7 3– anion is hydrogen bonded to its two adjacent neighbours forming infinite chains parallel to the b axis. Water molecules are located in infinite channels extended along the b direction. The oxygen atom of the water molecule is connected with two sodium cations and hydrogen bonded with one oxygen atom of diphosphate group.

Hydrothermal crystallization in the systems LiOH-MO2-GeO2-H2O (M = Ti, Sn) at 500°C and 0.1 GPa:Li2Ti[5]Ge[4]O5 and Li2Ge[6]Ge2[4]O6 phase relations

Crystallization in the LiОH–TiО2–GeО2–H2O and LiOH–SnO2–GeO2–H2O hydrothermal systems was studied at 500°C and 0.1 GPa. The phases crystallizing in the Ti system are Li2Ti[5]Ge[4]O5 , Li2Ge[6]Ge2[4]O6(OH)2 , Li3HGe4[6]Ge3[4]O16 · 4H2O, and Li2Ge[4]O3 (Ge in four- or sixfold oxygen coordination, and Ti in fivefold oxygen coordination). Over a wide range of TiO2 : GeO2 molar ratios (6 : 1 to 1 : 1), the dominant phase is Li2Ge[6]Ge2[4]O6(OH)2 . At higher GeO2 contents (TiO2 : GeO2 = 1 : 2 to 1 : 6), both Li2Ti[5]Ge[4]O5 and Li2Ge[6]Ge2[4]O6(OH)2 crystallize. The crystallization fields of Ge-containing phases at TiO2 : GeO2 = 1 : 2 to 1 : 6 are (in order of increasing LiOH concentration) GeO2 (rutile structure), Li2Ti[5]Ge[4]O5 + Li2Ge[6]Ge2[4]O6(OH)2, and Li2Ti[5]Ge[4]O5 + Li2Ge[6]Ge2[4]O6(OH)2 + Li2Ge[4]О3 + Li3HGe4[6]Ge3[4]O16 · 4H2O. The phases crystallizing in the Sn system are Li2Ge[6]Ge2[4]O6(OH)2, Li3HGe4[6]Ge3[4]O16 · 4H2O, and Li2Ge[4]O3. Tin is present in the form of SnO2 only. The structures of Li2Ti[5]Ge[4]O5 and Li2Ge[6]Ge2[4]O6(OH)2 are characterized by the simplest type of matrix assembly: direct packing of substructural precursor units and their two-dimensional growth in various crystallographic directions.

Surface X-ray structure analysis of the TiO 2(100)-(1 × 3) reconstruction

Using grazing incidence surface X-ray diffraction, we have investigated the superstructure of TiO 2(100)-(1 × 3) (rutile). The analysis of both fractional order superlattice rods and integer order crystal truncation rods (229 independent reflections in total), leads to a structure model which can basically be described as the formation of {110} micro-facets on the surface. In addition, strong lateral and vertical relaxations of the titanium and oxygen atoms exist down to about 9 Å below the surface. The surface titanium atoms are found to reside in different environments, namely in three-, five- and six-fold oxygen coordination and in a bridge site. Furthermore, we determined a 40% occupancy of an interstitial site occupied by one titanium atom per (1 × 3) unit cell. The surface stoichiometry is given by the formula TiO 1.68. Our structure model is significantly different from previous X-ray data [P. Zschack et al., Surf. Sci. 262 (1992) 395], but is in good agreement with spectroscopic experiments and scanning tunneling microscopy investigations.

Structural and electrical characterization of titania-doped YSZ

Solid solutions and composites based on YSZ (yttria-stabilized zirconia) with titania additions up to 20 mol% were prepared by solid state reaction. Structural characterization included XRD and Raman spectroscopy. Electroded samples were also studied by impedance spectroscopy in air between 573 and 1273 K. The ionic conductivity of these solid solutions decreases with increasing titania content. A sharp drop in conductivity is observed for specimens with compositions near to the usually assumed solubility limit of titania in the fluorite phase (about 10 mol%). This decrease is of almost one order of magnitude with respect to undoped or lightly doped samples. A smaller composition dependence is observed for higher titania additions (10–20 mol% TiO 2). Raman results show that tetragonal short range order is found in the cubic fluoritic phase well below the 10 mol% TiO 2 addition. This short range order is explained by the ability of Ti to achieve a sixfold oxygen coordination. As a consequence, a decrease in the concentration of mobile oxygen vacancies is expected, which is the reason for the electrical conductivity decrease.

Defect fluorite structures in the Bi-rich part of the system Bi 2O 3Re 2O 7

Bi-rich ternary oxides of the system Bi 2O 3Re 2O 7 with composition Bi 2 − x Re x O 3 + 2 x have crystal structures with fluorite-like subunit cells. Several superunit cells are detected for different Re contents by X-ray and electron diffraction: Materials around x = 0.05 are characterized by a √2 × √2 × 1 tetragonal superunit cell of the β-Bi 2O 3 type. For higher Re contents a β → δ phase transition takes place. Around x = 0.1 a 3 × 3 × 3 tetragonal superunit cell develops, whereas for x = 0.2, a rhombohedral distortion is present. According to the vibrational spectroscopic investigations, the Re guest ion enters the lattice as ReO 4 tetrahedra as well as with six-fold oxygen coordination. By diffuse reflectance spectroscopy it is shown that Re doping influences the band structure via the introduction of impurity levels within the band gap. Thus, the system Bi 2O 3Re 2O 7 is suitable for fine-tuning the band gap between ≈ 2.2 and 3.0 eV.

Diffusive Propagation of Phonon Beams in Yttrium Aluminum Garnets Containing Substitutional Rare Earth Atoms

The diffusive maxima of phonon signals are studied for a number of solid solutions of rare earth atoms in yttrium aluminum garnets. The used exact formula for the diffusion constants allows for qualitative discussion of the obtained results. The established energy of phonons, forming the diffusive maximum of phonon signal of the temperature TH arriving at the bolometer, ranges from 3.2kBTH to 4.2kBTH, which is in reasonable agreement with the existing estimations. The qualitative analysis allows us to estimate the contribution, made by the rare earth ions occupying the octahedral positions of the sixfold oxygen coordination, to the scattering of phonons due to lattice imperfections in yttrium aluminum garnets.

Lanthanum L3-, europium L3- and copper K-edge EXAFS investigation of lanthanum copper oxides

Some cation-substituted and oxygen-rich compounds derived from the lanthanum copper oxide of composition La2CuO4 have been examined by measurement of the copper K-, lanthanum L3- and europium L3-edge extended X-ray absorption fine structure. The results obtained from La2CuO4 are in good agreement with previously reported data and are consistent with the six-fold oxygen co-ordination of copper and nine-fold coordination by oxygen of lanthanum. The results obtained from the compound Eu2CuO4 show co-ordination numbers of four and eight for copper and europium as would be expected for a structure containing copper in a square-planar oxygen environment and europium in square antiprismatic oxygen co-ordination.

EPR studies of 241Am doped BaO: Stabilization of O −, O −2, Am 4+ AND Am 0

Electron paramagnetic resonance studies of 1% 241Am doped barium oxide powder have been carried out at 77 K. In freshly prepared samples a sextet with prominent m 1 dependent line widths (with g = 2.085 and A = 25 G) was observed besides a sharp sextet with g = 2.011 and A = 10 G. In addition the spectra due to O − 2 and O − radical ions were also observed The sextet with characteristic m 1 dependent line widths appears to be due to Am 4+ at Ba 2+ site in six-fold oxygen coordination. From the known hyperfine fields due to core polarisation of Am 2+ and Am 0, it is conjuctured that the sextet with A = 10 G is due to neutral americium atoms with configuration 5f 77 s 2-