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Clare P. Grey - One of the best experts on this subject based on the ideXlab platform.
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Probing Oxide-Ion Mobility in the Mixed Ionic–Electronic Conductor La2NiO4+δ by Solid-State 17O MAS NMR Spectroscopy
Journal of the American Chemical Society, 2016Co-Authors: David M. Halat, Riza Dervisoglu, Gunwoo Kim, Matthew T. Dunstan, Frédéric Blanc, Derek S. Middlemiss, Clare P. GreyAbstract:While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic-electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution (17)O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic (17)O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.
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probing oxide ion mobility in the mixed ionic electronic conductor la2nio4 δ by solid state 17O MAS NMR spectroscopy
Journal of the American Chemical Society, 2016Co-Authors: David M. Halat, Riza Dervisoglu, Gunwoo Kim, Matthew T. Dunstan, Frédéric Blanc, Derek S. Middlemiss, Clare P. GreyAbstract:While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic-electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution (17)O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic (17)O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.
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Probing brønsted acid sites in zeolite HY with low temperature 17O MAS NMR spectroscopy
From Zeolites to Porous MOF Materials - The 40th Anniversary of International Zeolite Conference Proceedings of the 15th International Zeolite Confere, 2007Co-Authors: Hua Huo, Luming Peng, Clare P. GreyAbstract:Abstract Bronsted acid sites play a key role in controlling the catalytic performances of acidic catalysts. A determination of the structure of the acid site is fundamental to the understanding acid strength in these systems. The O-H distances in zeolite HY and HZSM-5 extracted from 17O-1H REDOR NMR data acquired at room temperature are noticeably longer than the results from calculations due to the presence of some restricted motions at room temperature, such as zeolite framework vibrations and O-H librational motion. We present here our 17O-1H REDOR NMR results of zeolite HY at a lower temperature of 183 K, where some of these motions are frozen out. By comparing the line shapes obtained from simulations performed with the SIMPSON package with the experimental data, an O-H distance of about 0.97 ∼ 0.98 A was obtained, which is consistent with the previous ab-initio calculation results. The results indicate that low temperature REDOR NMR spectroscopy can provide estimates of the O-H distance, which should prove useful in understanding zeolite structure and acidity.
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Detecting different oxygen-ion jump pathways in Bi2WO6 with 1- and 2-dimensional 17O MAS NMR spectroscopy
Chemistry of Materials, 2005Co-Authors: Namjun Kim, Rose-noëlle Vannier, Clare P. GreyAbstract:Bismuth tungsten oxide (Bi2WO6) is an m = 1 Aurivillius phase and some of its doped compounds show moderately high oxygen ion conductivity. These materials are structurally related to the more disordered bismuth vanadium oxide Aurivillius phases (or BIMEVOXes), which exhibit extremely high conductivities. We demonstrate that a combination of one- and two-dimensional, variable-temperature, 17O MAS NMR spectroscopy may be used to probe ionic motion in the Nb5+-doped bismuth tungstates and resolve different oxide-ion conduction mechanisms that occur over a very wide range of different time scales (10-1 to 10-4 s). The use of 17O two-dimensional exchange spectroscopy dramatically increases the range of oxide-ion conductors that can be investigated by MAS NMR. Motion commences in the (WO4)n2n- perovskite layers, with slow exchange between the Bi2O22+ and (WO4)n2n- layers occurring at higher temperatures, as detected by 2D NMR. Simulation of the 1D 17O NMR spectra of Bi2W1-xNbxO6 with x = 0.05 and 0.1 shows an ...
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17O MAS NMR study of the oxygen local environments in the anionic conductors y2 b1 xb x 2o7 b b sn ti zr
Journal of Solid State Chemistry, 2003Co-Authors: Namjun Kim, Clare P. GreyAbstract:Abstract The local environments for oxygen in yttrium-containing pyrochlores and fluorites, Y2(B1−xB′x)2O7 (B=Ti, B′=Sn, Zr) are investigated by using solid state 17O MAS NMR spectroscopy. The quadrupolar coupling constants of the I= 5 2 nucleus, 17O are sufficiently small for these ionic oxides, that high-resolution spectra are obtained from the MAS spectra. Different oxygen NMR resonances are observed due to local environments with differing numbers of metal cations (Y3+, Sn4+, Ti4+ and Zr4+), allowing the numbers of different local environments to be quantified and cation mixing to be investigated. Evidence for pyrochlore-like local ordering is detected for Y2Zr2O7, which nominally adopts the fluorite structure.
Namjun Kim - One of the best experts on this subject based on the ideXlab platform.
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Structure of Amorphous Tantalum Oxide and Titania-Doped Tantala: 17O NMR Results for Sol–Gel and Ion-Beam-Sputtered Materials
Chemistry of Materials, 2011Co-Authors: Namjun Kim, Jonathan F StebbinsAbstract:Amorphous thin films of high-refractive index metal oxides such as Ta2O5 are critical in multilayer optical components such as the main mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), but their atomic-scale structures are not well-known. Amorphous, pure, and titania-doped tantalum oxides were prepared by sol–gel synthesis and by ion-beam sputtering. The samples were successfully enriched with the nuclear magnetic resonance (NMR)-active 17O isotope by heating them in 17O2 gas, and they were studied using 17O solid-state NMR. 17O MAS NMR spectra of pure tantala show two resonances that correspond to two- and three-coordinated oxygens, allowing the average oxygen coordination numbers to be estimated from the relative peak areas. The average coordination number for oxide ions seems to increase as tantala changes from amorphous to crystalline. Titania-doped tantalas, (Ta1–xTix)2O5–x with x = 0.10, 0.25, and 0.50, were also studied. At the lower two dopant levels, Ta and Ti cations ar...
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High temperature 17O MAS NMR study of calcia, magnesia, scandia and yttria stabilized zirconia
Solid State Ionics, 2007Co-Authors: Namjun Kim, Cheng-han Hsieh, Hong Huang, Fritz B. Prinz, Jonathan F StebbinsAbstract:Abstract High-resolution 17 O MAS NMR can provide unique constraints on local structure and oxide ion dynamics in conductive zirconia ceramics of interest for fuel cells and other technologies. We describe here NMR and bulk conductivity measurements for scandia, yttria, calcia, and magnesia stabilized zirconias, including MAS NMR spectra collected in situ at temperatures up to 700 °C. All of the cubic compounds with high dopant levels showed line narrowing and coalescence in this temperature range, and the temperature-induced changes in line widths are qualitatively correlated with the bulk conductivities. A monoclinic zirconia with 2% Sc 2 O 3 dopant level (expected to have relatively low ionic mobility) showed no motional averaging of its two 17 O NMR peaks even at 600 °C, but instead is observed to begin to transform to a disordered, possibly cubic or tetragonal phase at 600 to 700 °C. 17 O MAS NMR spectra of calcia stabilized zirconia were analyzed in detail and the exchange frequencies as a function of temperature, conductivity, and activation energy were estimated and compared with bulk conductivities. The activation energy estimated from NMR exchange frequencies is somewhat lower than that of bulk conductivity but the conductivities estimated from NMR appear to be lower than bulk conductivity.
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Vacancy and Cation Distribution in Yttria-Doped Ceria: An 89Y and 17O MAS NMR Study
Chemistry of Materials, 2007Co-Authors: Namjun Kim, Jonathan F StebbinsAbstract:The local structure and dynamics of oxide ions in yttria-doped ceria (YDC, 5 to 30% Y2O3) were studied using high-resolution 89Y and 17O MAS NMR spectroscopy at ambient temperature and high temperatures to 500 °C. Eight-, seven-, and six-coordinated yttrium cations are clearly resolved in 89Y MAS NMR spectra, and their relative populations were measured. The derived average coordination number of yttrium is smaller than that for a random distribution of oxygen vacancies, suggesting that there is strong association between yttrium cations and vacancies and there is the possibility of pairing of two yttrium cations with one vacancy. In the 17O MAS NMR spectra, resonances for oxygens with different coordination environments are resolved and are assigned to oxygens with different numbers of yttrium cations in the first coordination sphere. The relative intensities of the 17O resonances also deviate from those expected from a random distribution, again indicating possible pairing of yttrium cations. High-tempe...
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Detecting different oxygen-ion jump pathways in Bi2WO6 with 1- and 2-dimensional 17O MAS NMR spectroscopy
Chemistry of Materials, 2005Co-Authors: Namjun Kim, Rose-noëlle Vannier, Clare P. GreyAbstract:Bismuth tungsten oxide (Bi2WO6) is an m = 1 Aurivillius phase and some of its doped compounds show moderately high oxygen ion conductivity. These materials are structurally related to the more disordered bismuth vanadium oxide Aurivillius phases (or BIMEVOXes), which exhibit extremely high conductivities. We demonstrate that a combination of one- and two-dimensional, variable-temperature, 17O MAS NMR spectroscopy may be used to probe ionic motion in the Nb5+-doped bismuth tungstates and resolve different oxide-ion conduction mechanisms that occur over a very wide range of different time scales (10-1 to 10-4 s). The use of 17O two-dimensional exchange spectroscopy dramatically increases the range of oxide-ion conductors that can be investigated by MAS NMR. Motion commences in the (WO4)n2n- perovskite layers, with slow exchange between the Bi2O22+ and (WO4)n2n- layers occurring at higher temperatures, as detected by 2D NMR. Simulation of the 1D 17O NMR spectra of Bi2W1-xNbxO6 with x = 0.05 and 0.1 shows an ...
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17O MAS NMR study of the oxygen local environments in the anionic conductors y2 b1 xb x 2o7 b b sn ti zr
Journal of Solid State Chemistry, 2003Co-Authors: Namjun Kim, Clare P. GreyAbstract:Abstract The local environments for oxygen in yttrium-containing pyrochlores and fluorites, Y2(B1−xB′x)2O7 (B=Ti, B′=Sn, Zr) are investigated by using solid state 17O MAS NMR spectroscopy. The quadrupolar coupling constants of the I= 5 2 nucleus, 17O are sufficiently small for these ionic oxides, that high-resolution spectra are obtained from the MAS spectra. Different oxygen NMR resonances are observed due to local environments with differing numbers of metal cations (Y3+, Sn4+, Ti4+ and Zr4+), allowing the numbers of different local environments to be quantified and cation mixing to be investigated. Evidence for pyrochlore-like local ordering is detected for Y2Zr2O7, which nominally adopts the fluorite structure.
Mark Smith - One of the best experts on this subject based on the ideXlab platform.
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A multinuclear solid state NMR, density functional theory and X-Ray diffraction study of hydrogen bonding in Group I hydrogen dibenzoates
CrystEngComm, 2013Co-Authors: Gregory J. Rees, Andrew P. Howes, Dinu Iuga, Stephen Day, Alberth Lari, Mateusz B. Pitak, Simon J. Coles, Terry L. Threlfall, Mark E. Light, Mark SmithAbstract:An NMR crystallographic approach incorporating multinuclear solid state NMR (SSNMR), X-ray structure determinations and density functional theory (DFT) are used to characterise the H bonding arrangements in benzoic acid (BZA) and the corresponding Group I alkali metal hydrogen dibenzoates (HD) systems. Since the XRD data often cannot precisely confirm the proton position within the hydrogen bond, the relationship between the experimental SSNMR parameters and the ability of gauge included plane augmented wave (GIPAW) DFT to predict them becomes a powerful constraint that can assist with further structure refinement. Both the 1H and 13C MAS NMR methods provide primary descriptions of the H bonding via accurate measurements of the 1H and 13C isotropic chemical shifts, and the individual 13C chemical shift tensor elements; these are unequivocally corroborated by DFT calculations, which together accurately describe the trend of the H bonding strength as the size of the monovalent cation changes. In addition, 17O MAS and DOR NMR form a powerful combination to characterise the O environments, with the DOR technique providing highly resolved 17O NMR data which helps verify unequivocally the number of inequivalent O positions for the conventional 17O MAS NMR to process. Further multinuclear MAS and static NMR studies involving the quadrupolar 7Li, 39K, 87Rb and 133Cs nuclei, and the associated DFT calculations, provide trends and a corroboration of the H bond geometry which assist in the understanding of these arrangements. Even though the crystallographic H positions in each H bonding arrangement reported from the single crystal X-ray studies are prone to uncertainty, the good corroboration between the measured and DFT calculated chemical shift and quadrupole tensor parameters for the Group I alkali species suggest that these reported H positions are reliable.
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17O and 15N solid state NMR studies on ligand-assisted templating and oxygen coordination in the walls of mesoporous Nb, Ta and Ti oxides.
Journal of the American Chemical Society, 2008Co-Authors: Yuxiang Rao, Mark Smith, Thomas F. Kemp, Michel Trudeau, Dave M. AntonelliAbstract:A multinuclear solid state NMR approach is applied to four templated mesoporous oxides (silica, titania, niobia and tantala) to include 15N and 17O magic angle spinning (MAS) NMR and double resonance 15N−93Nb, 17O Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR). The templated samples were ramped in steps of 20 °C for 2 days up to typically 110 °C where the samples were left for 2−4 days. 15N MAS NMR shows that amines are the only species present in the TiO2, Nb2O5, and Ta2O5. In SiO2, amines are only present as a minor coordination (10 ± 2%), but there are several strong ammonium 15N resonances. The REAPDOR experiments show that the nitrogen interacts with niobium, confirming a ligand interaction between the Nb and N, as previously believed. In the case of silica, the amine is quaternized and there is apparently no interaction with the Si, suggesting a RNH3+ −O−Si- hydrogen-bonding interaction with the walls. 17O MAS NMR provides the clearest indication of the local wall structure. In the aged, templated samples in all cases only OM2 coordinations are present which is very different from the pure bulk oxides (apart from SiO2) and must be due to the effects of amine coordination at the metal centers. On removal of the template, these oxides behave differently, with Ta2O5 showing a mixture of OTa2 (85 ± 5%) and OTa3 (15 ± 5%) which is similar to the types of coordination found in the bulk oxide. The previously reported 17O MAS NMR data from heat-treated mesoporous niobia shows only ONb2, which is very highly ordered. In contrast for titania, the OTi2 coordination is immediately lost on removal of the template to be replaced by a mixture of OTi3 (60 ± 5%) and OTi4 (40 ± 5%), with the OTi4 becoming dominant above 250 °C, very different behavior from the corresponding bulk oxide. In summary, this NMR study shows that the local oxygen coordination in amine-templated mesoporous transition metal oxides is present as OM2 which is relatively rare in bulk oxides. The data indicates that the template interaction is largely controlled by the N−M dative bond to the wall, suppressing higher oxygen coordination numbers. Qualitatively it appears that the strength of this interaction varies greatly in the different mesoporous oxides.
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Experimental and Theoretical 17O NMR Study of the Influence of Hydrogen-Bonding on CO and O−H Oxygens in Carboxylic Solids
The journal of physical chemistry. A, 2006Co-Authors: Alan Wong, Kevin J. Pike, Robert D. Jenkins, Guy J. Clarkson, Tiit Anupõld, Andrew P. Howes, David H. G. Crout, Ago Samoson, Ray Dupree, Mark SmithAbstract:A systematic solid-state 17O NMR study of a series of carboxylic compounds, maleic acid, chloromaleic acid, KH maleate, KH chloromaleate, K2 chloromaleate, and LiH phthalate·MeOH, is reported. Magic-angle spinning (MAS), triple-quantum (3Q) MAS, and double angle rotation (DOR) 17O NMR spectra were recorded at high magnetic fields (14.1 and 18.8 T). 17O MAS NMR for metal-free carboxylic acids and metal-containing carboxylic salts show featured spectra and demonstrate that this combined, where necessary, with DOR and 3QMAS, can yield site-specific information for samples containing multiple oxygen sites. In addition to 17O NMR spectroscopy, extensive quantum mechanical calculations were carried out to explore the influence of hydrogen bonding at these oxygen sites. B3LYP/6-311G++(d,p) calculations of 17O NMR parameters yielded good agreement with the experimental values. Linear correlations are observed between the calculated 17O NMR parameters and the hydrogen bond strengths, suggesting the possibility of ...
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Transition metal atom sites in ternary ZrO2-TiO2-SiO2 xerogels
Journal of Sol-Gel Science and Technology, 2003Co-Authors: Gavin Mountjoy, David M. Pickup, Graham Wallidge, Mark Smith, Mark A. Holland, Philips N. Gunawidjaja, Robert J. NewportAbstract:There has been much work on the binary TiO2-SiO2 and ZrO2-SiO2 materials prepared by sol-gel because of the beneficial properties resulting from incorporation of Ti and Zr. In contrast the ternary TiO2-ZrO2-SiO2 xerogels have been relatively little studied. We report the results of a study of those xerogels having Zr:Ti:Si ratios of 5:15:80, 10:10:80 and 15:5:80 heated to 750°C and to 1000°C. The study includes X-ray diffraction, small angle X-ray scattering, X-ray absorption spectroscopy at Ti and Zr K-edges, and 17O MAS-NMR. The study has benefited from close comparison with similar previous studies of the binary systems. The metal atoms in the ternary systems are shown to be predominantly homogeneously mixed in the silica network, as observed for the respective binary systems. The clear exception is for the sample with a minority of Zr, which after heat treatment at 750°C shows the presence of phase separation attributed to the formation of an amorphous precursor of ZrTiO4; at 1000°C this phase crystallises. In samples with higher Zr content the crystallisation of a ZrO2 tetragonal phase was observed. The data obtained illustrate well the strength of a research methodology in which a common batch of samples is studied using a coherent suite of modern structural probes.
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Structure of (Ta2O5)x(SiO2)1 −x xerogels (x = 0.05, 0.11, 0.18, 0.25 and 1.0) from FTIR, 29Si and 17O MAS NMR and EXAFS
Journal of Materials Chemistry, 2000Co-Authors: David M. Pickup, Gavin Mountjoy, Graham Wallidge, Robert J. Newport, Mark A. Holland, Mark SmithAbstract:A combination of 29Si and 17O MAS NMR, EXAFS and FT-IR spectroscopy has been used to study the atomic structure of (Ta2O5)x(SiO2)1 − x (x = 0.05, 0.11, 0.18 and 0.25) xerogels prepared by reacting partially-hydrolysed tetraethyl orthosilicate with tantalum(V) ethoxide. Amorphous tantala, a-Ta2O5, xerogels have also been prepared and their structures studied in detail for the first time. Results have shown that in all these materials, Ta adopts predominantly 5-fold coordination with respect to oxygen. For the mixed oxide xerogels, partial phase separation of the two component oxides occurs for x > 0.11.
Jonathan F Stebbins - One of the best experts on this subject based on the ideXlab platform.
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Non-stoichiometric non-bridging oxygens and five-coordinated aluminum in alkaline earth aluminosilicate glasses: Effect of modifier cation size
Journal of Non-Crystalline Solids, 2012Co-Authors: Linda M. Thompson, Jonathan F StebbinsAbstract:Both non-bridging oxygen (NBO) and high-coordinated aluminum (commonly VAl at atmospheric pressure) are believed to play important roles in the thermodynamic and transport properties of aluminosilicate melts but their changes with composition are not well understood, particularly in compositions that are charge-balanced or contain excess alumina. Here we present high-resolution 27Al and 17O MAS NMR data on barium aluminosilicate glasses, similar to previously studied calcium and potassium aluminosilicate glasses, allowing us to separate the effect of cation size versus cation charge. The NBO content decreases with increasing Al content but shows no significant difference compared to similar calcium aluminosilicate glasses. As there is a significant difference between similar potassium and calcium aluminosilicate glasses, this indicates that cation charge may be the important parameter in determining the amount of NBO present on the charge-balanced join. The VAl content increases with increasing Al content but is significantly lower than similar calcium aluminosilicate glasses. This data, together with the data from potassium and calcium aluminosilicate glasses, agrees with the well-known effect of increasing high coordinated aluminum with higher cation field. In contrast, the lack of changes observed in the “non-stoichiometric” NBO content despite the changes in the VAl content provides additional evidence to suggest that the formation of NBO and VAl is not linked.
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Structure of Amorphous Tantalum Oxide and Titania-Doped Tantala: 17O NMR Results for Sol–Gel and Ion-Beam-Sputtered Materials
Chemistry of Materials, 2011Co-Authors: Namjun Kim, Jonathan F StebbinsAbstract:Amorphous thin films of high-refractive index metal oxides such as Ta2O5 are critical in multilayer optical components such as the main mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), but their atomic-scale structures are not well-known. Amorphous, pure, and titania-doped tantalum oxides were prepared by sol–gel synthesis and by ion-beam sputtering. The samples were successfully enriched with the nuclear magnetic resonance (NMR)-active 17O isotope by heating them in 17O2 gas, and they were studied using 17O solid-state NMR. 17O MAS NMR spectra of pure tantala show two resonances that correspond to two- and three-coordinated oxygens, allowing the average oxygen coordination numbers to be estimated from the relative peak areas. The average coordination number for oxide ions seems to increase as tantala changes from amorphous to crystalline. Titania-doped tantalas, (Ta1–xTix)2O5–x with x = 0.10, 0.25, and 0.50, were also studied. At the lower two dopant levels, Ta and Ti cations ar...
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High temperature 17O MAS NMR study of calcia, magnesia, scandia and yttria stabilized zirconia
Solid State Ionics, 2007Co-Authors: Namjun Kim, Cheng-han Hsieh, Hong Huang, Fritz B. Prinz, Jonathan F StebbinsAbstract:Abstract High-resolution 17 O MAS NMR can provide unique constraints on local structure and oxide ion dynamics in conductive zirconia ceramics of interest for fuel cells and other technologies. We describe here NMR and bulk conductivity measurements for scandia, yttria, calcia, and magnesia stabilized zirconias, including MAS NMR spectra collected in situ at temperatures up to 700 °C. All of the cubic compounds with high dopant levels showed line narrowing and coalescence in this temperature range, and the temperature-induced changes in line widths are qualitatively correlated with the bulk conductivities. A monoclinic zirconia with 2% Sc 2 O 3 dopant level (expected to have relatively low ionic mobility) showed no motional averaging of its two 17 O NMR peaks even at 600 °C, but instead is observed to begin to transform to a disordered, possibly cubic or tetragonal phase at 600 to 700 °C. 17 O MAS NMR spectra of calcia stabilized zirconia were analyzed in detail and the exchange frequencies as a function of temperature, conductivity, and activation energy were estimated and compared with bulk conductivities. The activation energy estimated from NMR exchange frequencies is somewhat lower than that of bulk conductivity but the conductivities estimated from NMR appear to be lower than bulk conductivity.
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Vacancy and Cation Distribution in Yttria-Doped Ceria: An 89Y and 17O MAS NMR Study
Chemistry of Materials, 2007Co-Authors: Namjun Kim, Jonathan F StebbinsAbstract:The local structure and dynamics of oxide ions in yttria-doped ceria (YDC, 5 to 30% Y2O3) were studied using high-resolution 89Y and 17O MAS NMR spectroscopy at ambient temperature and high temperatures to 500 °C. Eight-, seven-, and six-coordinated yttrium cations are clearly resolved in 89Y MAS NMR spectra, and their relative populations were measured. The derived average coordination number of yttrium is smaller than that for a random distribution of oxygen vacancies, suggesting that there is strong association between yttrium cations and vacancies and there is the possibility of pairing of two yttrium cations with one vacancy. In the 17O MAS NMR spectra, resonances for oxygens with different coordination environments are resolved and are assigned to oxygens with different numbers of yttrium cations in the first coordination sphere. The relative intensities of the 17O resonances also deviate from those expected from a random distribution, again indicating possible pairing of yttrium cations. High-tempe...
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o atom sites in natural kaolinite and muscovite 17O MAS and 3qMAS NMR study
American Mineralogist, 2003Co-Authors: Jonathan F StebbinsAbstract:The layer silicates are among the most common minerals in the Earth’s surface environment, play important roles in many geological processes, and have diverse technological applications. While it has been suggested that O isotope exchange and dissolution kinetics in aqueous solutions are controlled by chemical bonding and local atomic structures, the effect of atomic environment around O atom sites in clay minerals on their site-specific reactivities with H2O are not well known, mainly because direct experimental evidence is lacking. Here, we present for the first time detailed high-resolution 17O NMR data for 17O-exchanged natural kaolinite [Al2Si2O5(OH)4] and muscovite [KAl2(AlSi3)O10(OH)2] using 17O triple quantum magic angle spinning (3QMAS) and MAS NMR at high fields. At least two basal O atom sites in kaolinite are resolved: O4, and (O3 + O5). Apical O atoms ([4]Si-O-2[6]Al) and hydroxyl groups are also shown in these spectra. The 17O 3QMAS spectrum for muscovite shows improved resolution over the 17O MAS NMR spectrum, allowing us to resolve several basal O atoms, including ([4]Si-O-[4]Al), as well as hydroxyl groups. The fraction of each O atom appears to deviate somewhat from the stoichiometric value, suggesting that each crystallographically distinct site may have a different rate of exchange with the O atom in H2O.
Laurent Delevoye - One of the best experts on this subject based on the ideXlab platform.
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Natural abundance 17O MAS NMR and DFT simulations: New insights into the atomic structure of designed micas.
Solid state nuclear magnetic resonance, 2019Co-Authors: Esperanza Pavón, Francisco J. Osuna, María D. Alba, Laurent DelevoyeAbstract:Abstract Combining 17O Magic-Angle Spinning (MAS) NMR at natural abundance with DFT calculations is a promising methodology to shed light on the structure and disorder in tetrahedral sheets of designed micas with enhanced properties. Among brittle micas, synthetic mica is an important alternative to natural ones with a swelling sheet-like structure that results in many applications, by exploiting unique characteristics. Lowenstein's rule is one of the main chemical factor that determines the atomic structure of aluminosilicates and furthermore their properties. In the present article, 17O MAS NMR spectroscopy is used to validate (or not) the agreement of the Lowenstein's rule with the distribution of Si and Al sites in the tetrahedral sheets of synthetic micas. 17O MAS spectra of synthetic high-charged micas exhibit two regions of signals that revealed two distinguishable oxygen environments, namely Si-O-X (with X = Si, Altet, Mg) and Altet-O-Y (Y=Mg or Altet). DFT calculations were also conducted to obtain the 17O chemical shift and other NMR features like the quadrupolar coupling constant, CQ, for all of the oxygen environments encountered in the two model structures, one respecting the Lowenstein's rule and the other involving Altet-O-Altet and Si-O-Si environments. Our DFT calculations support the 17O assignment, by confirming that Altet-O-3Mg and Altet-O-Altet oxygen environments show chemical shifts under 30 ppm and more important, with quadrupolar coupling constants of about 1 MHz, in line with the spectral observation. By quantifying the 17O MAS NMR spectra at natural abundance, we demonstrate that one of the synthetic mica compositions does not meet the Lowenstein's rule.
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17O MAS NMR studies of oxo based olefin metathesis catalysts a critical assessment of signal enhancement methods
Physical Chemistry Chemical Physics, 2016Co-Authors: Denys Grekov, Iker Del Rosal, Mostafa Taoufik, Regis M Gauvin, Yassine Bouhoute, Laurent Maron, Laurent DelevoyeAbstract:The DFS enhancement method as applied to 17O MAS NMR was critically assessed, first on NaPO3, a simple binary glass system, and in a second step, on a series of catalysis-related organometallic molecules and materials. The robustness of DFS was investigated for the wide range of anisotropic parameters (quadrupolar coupling and chemical shift anisotropy) encountered in these samples. Emphasis has been put on the variation of signal enhancements with respect to the DFS final sweep frequency, pulse amplitude and pulse duration, while line shape distortion issues were also addressed. Finally, the robustness of DFS enhancement of the 17O MAS NMR signal is shown through its successful application to silica-supported olefin metathesis catalysts.
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17 O MAS NMR studies of oxo-based olefin metathesis catalysts: a critical assessment of signal enhancement methods
Physical Chemistry Chemical Physics, 2016Co-Authors: Denys Grekov, Mostafa Taoufik, Yassine Bouhoute, Laurent Maron, Iker Del rosal, Régis Gauvin, Laurent DelevoyeAbstract:The DFS enhancement method as applied to 17O MAS NMR was critically assessed, first on NaPO3, a simple binary glass system, and in a second step, on a series of catalysis-related organometallic molecules and materials. The robustness of DFS was investigated for the wide range of anisotropic parameters (quadrupolar coupling and chemical shift anisotropy) encountered in these samples. Emphasis has been put on the variation of signal enhancements with respect to the DFS final sweep frequency, pulse amplitude and pulse duration, while line shape distortion issues were also addressed. Finally, the robustness of DFS enhancement of the 17O MAS NMR signal is shown through its successful application to silica-supported olefin metathesis catalysts.
