Oxygen Coordination Environment Research Articles

93Nb NMR chemical shift scale for niobia systems

93Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the 93Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the 93Nb NMR chemical shift scale for NbO x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950 ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980 ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360 ppm; the shifts from −1200 to −1600 ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400 ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO x –MO y polyhedra association has been suggested. The magnitude of the 93Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz.

An equation of state for silicate melts. III. Analysis of stoichiometric liquids at elevated pressure: shock compression data, molecular dynamics simulations and mineral fusion curves

Experimental data and molecular dynamics simulations that constrain the densities of liquids of “mineral-like” stoichiometry at elevated pressure are evaluated using the liquid Equation of State (EOS) of Ghiorso (2004a). Phase equilibrium constraints on melt density are developed from experimental brackets on mineral fusion curves, specifically congruent melting of albite, anorthite, cristobalite/quartz/stishovite, diopside, enstatite, fayalite, forsterite, jadeite, nepheline, pyrope, sanidine, and titanite. A self consistent thermodynamic analysis of shock compression is applied to experimentally determined brackets on the Hugoniot for the liquid compositions CaMgSi₂O₆, CaAl₂Si₂O₈, Fe₂SiO₄, and (CaMgSi₂O₆)_0.64(CaAl₂Si₂O₈)_0.36. Molecular dynamics (MD) simulations of both melt density and melt structure are analyzed utilizing the liquid EOS of Ghiorso (2004a) and an ideal associated solution model that accounts explicitly for the effect of pressure and temperature on the oxygen coordination environment of silicon and aluminum. MD data on Mg₂Si₂O₆, CaMgSi₂O₆, SiO₂, Na₂Si₄O₉, CaAl₂Si₂O₈, NaAlSiO₄, NaAlSi₂O₆, and NaAlSi₃O₈ liquids are considered in conjunction with the fusion curve and shock compression experimental data sets. An internally consistent assessment of liquid EOS parameters from all available data sources is attempted for each liquid composition considered. Shock compression experiments on more chemically complex liquids of komatiite and MORB bulk composition are also examined. In support of the analysis of mineral fusion curves, volumetric properties as a function of temperature and pressure of diopside, enstatite (both <i>Pbca</i> and <i>C2/c</i>), fayalite, forsterite, and pyrope are evaluated from literature data. Experimental observations are parameterized to a Universal EOS (Vinet and others, 1986, 1987, 1989). It is found that the reference pressure properties derived by Ghiorso and Kress (2004) are internally consistent with the majority of high-pressure constraints on melt density. The notable exceptions are molten SiO₂ and melts in the system MgO-SiO₂. For the latter system, the calibration of Ghiorso and Kress (2004) fails to recover reference pressure volumes. The effect of configurational collapse (the volumetric response associated with changes in melt structure, such as changing oxygen coordination number about a cation) is systematically assessed from molecular dynamics simulation data. Fully depolymerized melts exhibit an ∼11 percent density increase associated with Si and Al transforming from IV- to V-fold coordination with respect to oxygen. Alkali aluminosilicate melts demonstrate ∼27 percent increase in melt density and SiO₂ and Na₂Si₄O₉ liquids show a density increase of ∼17 percent.

Structure—Property Relationships in the Nonlinear Optical Crystal KTiOPO4 Investigated Using NMR and ab Initio DFT Calculations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The complete 51V MAS NMR spectrum of surface vanadia nanoparticles on anatase (TiO2): vanadia surface structure of a DeNOx catalyst.

The first observations of the complete manifold of spinning sidebands (ssbs) including both the central and satellite transitions in (51)V MAS NMR spectra of surface vanadia nanoparticles on titania in DeNO(x) catalysts are presented. (51)V quadrupole coupling and chemical shift anisotropy parameters for the dominating vanadia structure are determined from (51)V MAS NMR spectra recorded at 9.4 and 14.1 T. Based on correlations previously established between (51)V NMR parameters and crystal structure data for inorganic vanadates, the NMR data are consistent with vanadium in a distorted octahedral oxygen coordination environment for the so-called strongly bonded vanadia species on the surface. The investigation includes two vanadia-titania model catalysts and six industrial-type DeNO(x) catalysts.

Structure−Property Relationships in the Nonlinear Optical Crystal KTiOPO4Investigated Using NMR and ab Initio DFT Calculations

17O, 39K, and 47,49Ti solid-state NMR studies of the nonlinear optical crystal potassium titanyl phosphate, KTiOPO4 (KTP), are reported. The results are compared with ab initio calculations using the WIEN2k program and discussed in specific relation to the bond-polarizability approach for the calculation of second-order nonlinear optical properties. It is found that the oxygen atoms in the anomalously short bonds to the titanium atoms (denoted by OT) have a considerably more ionic environment than the oxygen atoms (denoted by OP) that are involved in bridging TiO6 octahedra and PO4 tetrahedra. This result is consistent with the fractional covalencies computed using bond-polarizability principles for the Ti−OT and Ti−OP bonds, respectively. The NMR experiments are shown to validate independently the bond-polarizability approach to the calculation of nonlinear activity in KTP and demonstrate conclusively that the original notion that strong covalency in the short Ti−OT bonds is the principal source of nonlinear activity is incorrect. The field gradients at the Ti sites are estimated from NMR experiments to be ∼40 MHz, in agreement with WIEN2k calculations. These values are much larger than any found in previous studies of titanates and cannot be explained by models based on the distortion of the local octahedral oxygen coordination environment. It is suggested that both the large field gradient at the Ti sites and the variation of characteristics displayed by the different oxygen atoms in the structure may arise from other significant interactions, such as delocalization of charge or charge transfer, in significant structural chains of polyhedral units.

Systematic empirical analysis of calcium–oxygen coordination environment by calcium K-edge XANES

The X-ray absorption near edge structure (XANES) at the calcium K-edge is rich in information, but complex and difficult to interpret fully. We present here a systematic study of a range of calcium/oxygen containing compounds and minerals and show that the XANES may be used to obtain qualitative information on the calcium coordination environment.

Vanadium‐51 MAS and Static NMR Studies of the Binary V2O5—WO3 System.

Solid-state NMR spectra showing single-quantum excitation of the vanadium-51 (I = 7/2) central and six satellite transitions were obtained for polycrystalline V2O5 and x%V2O5−(100 − x%)WO3 (10 mol % ≤ x ≤ 40 mol %) solids by magic angle spinning (MAS) and static NMR spectroscopies. All MAS spectra show full spinning sideband manifolds resulting from incomplete averaging of the chemical shift and quadrupolar interaction anisotropies. Chemical shift parameters (δiso, δaniso, ηcs) and quadrupolar parameters (CQ, ηQ) were obtained by simulation of both MAS and static spectra. A single vanadium site was observed in the nominally 40 mol % V2O5−60 mol % WO3 and 30 mol % V2O5−70 mol % WO3 compositions. Interpretation of NMR spectral parameters suggests that the vanadium in this single site resides in a square pyramid-like oxygen coordination environment. Three sites were observed for the nominally 20 mol % V2O5−80 mol % WO3 and 10 mol % V2O5−90 mol % WO3 compositions. The spectral parameters for the three sites a...

Vanadium-51 MAS and Static NMR Studies of the Binary V2O5−WO3 System

Solid-state NMR spectra showing single-quantum excitation of the vanadium-51 (I = 7/2) central and six satellite transitions were obtained for polycrystalline V2O5 and x%V2O5−(100 − x%)WO3 (10 mol % ≤ x ≤ 40 mol %) solids by magic angle spinning (MAS) and static NMR spectroscopies. All MAS spectra show full spinning sideband manifolds resulting from incomplete averaging of the chemical shift and quadrupolar interaction anisotropies. Chemical shift parameters (δiso, δaniso, ηcs) and quadrupolar parameters (CQ, ηQ) were obtained by simulation of both MAS and static spectra. A single vanadium site was observed in the nominally 40 mol % V2O5−60 mol % WO3 and 30 mol % V2O5−70 mol % WO3 compositions. Interpretation of NMR spectral parameters suggests that the vanadium in this single site resides in a square pyramid-like oxygen coordination environment. Three sites were observed for the nominally 20 mol % V2O5−80 mol % WO3 and 10 mol % V2O5−90 mol % WO3 compositions. The spectral parameters for the three sites a...

Simultaneous Probing of the Fe and Co Sites in the CO2-Absorber Perovskite Sr0.95Ca0.05Co0.5Fe0.5O3-δ: a Mössbauer Study

The Co and Fe sites have been probed in pristine and CO2-treated Sr0.95Ca0.05Fe0.5Co0.5O3-δ using emission (EMS) and transmission (TMS) Mossbauer spectroscopy and found to have different local oxygen coordination environments and, most probably, a layered arrangement in the lattice. The EMS and TMS spectra of the pristine material could be evaluated with a three-doublet model, with only one of the doublets being different in the two cases. The doublets were assigned to Fe species with localized and delocalized electronic states. Absorption of CO2 at 650 °C resulted in magnetic ordering, which appeared to be static in EMS and rather dynamic in TMS spectra. This difference was attributed to the lower average oxygen coordination number of Co, and therefore ready absorption of CO2 at O vacancies. CO2 treatment at 950 °C triggered partial phase separation, confirmed also by scanning electron microscopy and X-ray microanalysis measurements. It was concluded that absorption of CO2 occurs preferentially in Co-ric...

An extended X-ray absorption fine structure spectroscopy investigation of cadmium sorption on cryptomelane (KMn 8O 16)

The mobility of cadmium in the environment is strongly inhibited by sorption onto Fe and Mn (hydr)oxide minerals such as cryptomelane (KMn 8O 16). Adsorption experiments showed that cryptomelane was able to sorb two thirds of available cadmium from solution at pH as low as 2.0. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to determine whether cadmium had sorbed to the external surfaces of the mineral, or migrated into the ∼4.6 Å diameter tunnels which exist in the cryptomelane structure. The Mn coordination environment around sorbed cadmium (4.9±1.0 Mn at 3.65±0.02 Å) at pH 2.0 strongly indicates that the majority of sorbed cadmium is located inside the tunnels, and that it is displaced from the ideal tunnel cation position (special position 2 a (0, 0, 0)). The oxygen coordination environment around cadmium (6.5±1.3 O at 2.24±0.02 Å) is consistent with this conclusion, but also suggests that sorbed cadmium is partially hydrated. The dominant tunnel cation (K +) was not significantly released to solution during cadmium sorption. Thus, it seems likely that cadmium exchanged with H + in the tunnels rather than K +. This is supported by the lowering of pH during cadmium adsorption and corresponding charge balance calculations. Cadmium sorption at the tunnel sites is likely to be energetically favourable because it allows occupation of those tunnel sites which K + cannot fill, thus resulting in a more effective balancing of the negative structural charge in cryptomelane. This is the first EXAFS study of cation sorption on a mineral with the hollandite structure.

Mid-IR and far-IR investigation of AgI-doped silver diborate glasses.

The structures of $x\mathrm{AgI}+(1\ensuremath{-}x){\mathrm{Ag}}_{2}\mathrm{O}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glasses, where $0.2l~xl~0.6$, have been investigated using mid-and far-infrared spectroscopy. The mid-IR spectra revealed that in those glasses prepared using AgN${\mathrm{O}}_{3}$ as the starting material for ${\mathrm{Ag}}_{2}$O, the B${\mathrm{O}}_{4}^{\ensuremath{-}}$/B${\mathrm{O}}_{3}$ ratio is constant with increasing amounts of AgI as would be expected form the proposed behavior of AgI in these glasses. However, a survey of the literature revealed those glasses prepared from pure ${\mathrm{Ag}}_{2}$O show a strong linear dependence of the B${\mathrm{O}}_{4}^{\ensuremath{-}}$/B${\mathrm{O}}_{3}$ ratio on AgI content. Most probably, in those glasses prepared with ${\mathrm{Ag}}_{2}$O the ${\mathrm{Ag}}_{2}$O/${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ ratio changes with AgI content due to the decomposition of ${\mathrm{Ag}}_{2}$O during melting. This different behavior is associated with AgN${\mathrm{O}}_{3}$ decomposing to ${\mathrm{Ag}}_{2}$O with heating followed by incorporation into the glassy network. For ${\mathrm{Ag}}_{2}$O used directly, it is proposed that it decomposes to Ag metal and ${\mathrm{O}}_{2}$(gas) with heating before it can be incorporated into the borate network. This latter behavior decreases with increasing AgI in the batch composition because AgI lowers the liquidus temperature of the melt considerably. The far-IR analysis of the AgI-doped silver diborate glasses suggests that there are three coordination environments for the ${\mathrm{Ag}}^{+}$ ions; one with iodide anions and the other two with oxygen ions. It is proposed that the separate oxygen coordination environments for the ${\mathrm{Ag}}^{+}$ ions arise from one with bridging oxygens of B${\mathrm{O}}_{4}^{\ensuremath{-}}$ units, and the other with nonbridging oxygens on B${\mathrm{O}}_{3}^{\ensuremath{-}}$ units. Furthermore, it is proposed that the ${\mathrm{Ag}}^{+}$ ions in the iodide-ion environments progressively agglomerate into disordered regions of AgI, but do not form structures similar to $\ensuremath{\alpha}$-AgI. These results appear to support the conduction pathway or microdomain model for ionic conduction in $x\mathrm{AgI}+(1\ensuremath{-}x){\mathrm{Ag}}_{2}\mathrm{O}\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{B}}_{2}{\mathrm{O}}_{3}$ glasses where the pathways are built up form disordered structures of AgI.

Redetermination of the structure of Gd2CuO4: a site population analysis

MoO2DPO4.D20 Table 2. Bond distances (A)and angles (o) with e.s.d.'s i in parentheses -~-'L-~ :i ol oa Mo Mo--Ol Mo-O2 Mo--O4 Mo-O5 Mo-O6 DI-O4 D2--O3 O4--Mo-O6 O6--Mo--O2 O4--Mo-O5 O2-Mo-O5 O 1--Mo-O2 O l--Mo-O4 O l--Mo-O5 OI--Mo-O6 Fig. 2. Portion of the structure MoO2DPO4.D20 viewed obliquely to the y = 0.25 plane. The hydrogen bonds are represented by broken lines. positions determined were significantly displaced from those obtained by Kierkegaard. The Mo environment consists of two short bonds to 0 4 and 06 typical of a bent molybdenyl group, three normal M o - O bonds linking to phosphate groups and a more weakly bound D20 molecule with an M o - O distance of 2.27 A. The phosphate group is protonated as is found in other transition-metal phosphates, for example Zr(HPO4)2.- H20. The P - O - H group hydrogen bonds reasonably strongly to 04, which is the sole interaction between the double chains. The water-molecule geometry is typical for this species in hydrated inorganic compounds; it is held in the y = 0.25 plane by a weak hydrogen bond between D2 and 03. The presence of this structural feature fits with the strong absorption at 1620cm -t in the IR spectrum which is due to the H20 symmetric bend. The coordination geometry of Mo is similar to that observed in molecular Mo[(CH3)2NCHO]202CI 2 (Flo- P--Ol P--O2 P--O3 O3--DI O5-D2 O5--D3 D2-D3 D2-O5-D3 P--O3--D I O3--D I--O4 O5-D2-O3 O I--P--O2 O 1-P--O3 O2-P--O3 I 12.9 rian & Corey, 1968) where a bent M o O 2 group' coordinated to oxygen and chlorine in an approxi- mately octahedral environment. In MoO2DPO4.D20, however, the phosphate groups link the molybdenyl moieties into infinite chains. The authors wish to thank the ILL, Grenoble, for the use of neutron beam facilities. We also thank the SERC for a grant for work in this field and a studentship for RGB. References FLORIAN, L. R. & COREY, E. R. (1968). Inorg. Chem. 7, 722-725. HAMILTON, W. C. (1965). Acta Cryst. 18, 502-510. HEWAT, A. W. (1973). Report AERE-R7350. UK Atomic Energy Authority, Harwell, Oxon, England. KmRKEGAARD, P. (1958). Acta Chem. Scand. 12, 1701-1714. KmRrmGAARD, P. (1962). Ark. Kemi, 19, 51-62. RETVELD, H. M. (1969). J. Appl. Cryst. 2, 65-71. SCnULYZ, I. (1955). Z. Anorg. Allg. Chem. 281, 99-112. Acta Cryst. (1988). C44, 1518-1520 R e d e t e r m i n a t i o n o f the Structure o f Gd 2CUO4: A Site P o p u l a t i o n A n a l y s i s BY KIMBERLY A. KUBAT-MARTIN,* ZACHARY FISKt AND ROBERT R. RYAN* Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545, USA (Received 10 February 1988; accepted 25 April 1988) Abstract. In view of recent interest in compounds of the type Ln2CuO 4 (where Ln = lanthanide), a single-crystal X-ray study has been performed for the redeter- mination of the structure of Gd2CuO 4 including a site population analysis. M r = 442.04, tetragonal, I4/mmm, * Group INC-4, MS C346. I Group P10, MS K764. a = 3.892 (1), c = 11.878 (3) A, V = 179.91 A 3, Z = 2, D x = 8 . 1 5 g c m -a, MoKa~, 2 = 0 . 7 0 9 2 6 A , # = 423.5 cm -~, F(000) = 378, T = 295 K, R = 2.8%, 102 unique reflections used for refinement. The structure consists of a two-dimensional edge-linked square-planar network of [CuO2] 2- groups which are linked by planes of Gd, O and Gd atoms. The oxygen coordination environment around the Gd 3+ cations is cubic. The © 1988 International Union of Crystallography

Theory of order-disorder transformation in binary systems of the Cu-Au type — I

LiNaCo0.5Fe0.5PO4F fluoride-phosphate was synthesized via conventional solid-state and novel freeze-drying routes. The crystal structure was refined based on neutron powder diffraction (NPD) data and validated by electron diffraction (ED) and high-resolution transmission electron microscopy (HRTEM). The alkali ions are ordered in LiNaCo0.5Fe0.5PO4F and the transition metals jointly occupy the same crystallographic sites. The oxidation state and oxygen coordination environment of the Fe atoms were verified by 57Fe Mössbauer spectroscopy. Electrochemical tests of the LiNaCo0.5Fe0.5PO4F cathode material demonstrated a reversible activity of the Fe3+/Fe2+ redox couple at the electrode potential near 3.4 V and minor activity of the Co3+/Co2+ redox couple over 5 V vs Li/Li+. The material exhibited the discharge capacity of more than 82% (theo.) regarding Fe3+/Fe2+ in the 2.4÷4.6 V vs Li/Li+ potential range.