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R Dupree

Publications and source records attributed to R Dupree.

13 recordsLinked to original sources
PubMed2005

A 125Te and 23Na NMR investigation of the structure and crystallisation of sodium tellurite glasses.

125Te static nuclear magnetic resonance (NMR) and 23Na and 125Te magic angle spinning (MAS) NMR have been used, in conjunction with X-ray diffraction, to examine the structure and crystallisation behaviour of glasses of composition xNa2O.(1-x)TeO2 (0.075 x 0.4). The MAS NMR 23Na spectra from the glasses are broad and featureless but shift by approximately +5 ppm with increased x, i.e. as the system becomes more ionic. The static 125Te NMR spectra show an increase in axial symmetry with increasing x, indicating a shift from predominantly [TeO4] to [TeO3] structural units. The 23Na and 125Te spectra from the crystallised samples have been fitted to obtain information on the sites in the metastable crystal phases, which are the first to form on heating and which are therefore more closely related to the glass structure than thermodynamically stable crystal phases. New sodium tellurite phases are reported, including a sodium stabilised, face centred cubic phase related to delta-TeO2; a metastable form of Na2Te4O9 containing 3 sodium and 4 tellurium sites; and a metastable form of Na2Te2O5 containing 2 sodium sites. There is evidence of oxidation of TeIV to TeVI occurring in glasses with high values of x and, at x=0.40 and 0.50 (outside the glass forming range), some sodium metatellurate (Na2TeO4) is formed at the same time as sodium metatellurite (Na2TeO3). The 125Te shift is very sensitive to environment within the sodium tellurite system, covering more than 320 ppm, with anisotropies varying from 640 to 1540 ppm. The lack of features in the 125Te spectra of the glass phases, combined with the large shift range and high but variable anisotropy, means than it is not possible to obtain a unique fit to any presumed species present. Furthermore, the chemical shift anisotropy parameters for three of the four Te sites in the Na2Te4O9 phase are found to lie outside the range used for previous simulations of glass spectra.

Journal Article↗
PubMed2000

H2O/OH ratio determination in hydrous aluminosilicate glasses by static proton NMR and the effect of chemical shift anisotropy.

Static 1H NMR spectra of hydrous NaAlSi3O8 glasses have been acquired at low temperature (140 K) in order to quantitatively determine OH and H2O concentrations. Since both components overlap in the spectra, an unambiguous determination of the line shapes is required. The structurally bonded hydroxyl groups are well described by a Gaussian line and the water molecules exhibit a Pake doublet-like line shape due to the strong proton-proton dipolar interaction. However, at proton resonance frequencies used in this study (360 MHz), the Pake doublet has an asymmetric line shape due to chemical shift anisotropy (CSA), which is significant and must be included in any simulation in order to reproduce the experimental line shape successfully. The simulations for rigid water molecules dissolved in our hydrous aluminosilicate glasses result in a CSA of 30+/-5 ppm and a dipolar interaction constant of 63.8+/-2.5 kHz (i.e., dipolar coupling constant (DCC) of 42.6+/-1.7 kHz), corresponding to a proton-proton distance of r(ij) = 154+/-2 pm. In contrast to earlier work, water speciation obtained from the simulations of our 1H NMR spectra are in excellent agreement with those obtained from infrared (IR) spectroscopy.

Aluminum Silicates↗
PubMed2000

Determination of titanium NMR parameters of ATiO3 compounds: correlations with structural distortion.

Solid state 47,49Ti NMR spectra have been obtained for a number of perovskite and ilmenite ATiO3 compounds. The 49Ti quadrupole coupling constant varies from 2.75 MHz (CaTiO3) to 15.5 MHz (MgTiO3) and the electric field gradient at the titanium site was found to correlate well with the shear strain, independent of structure. The chemical shift in the perovskite structures varies by approximately 160 ppm and correlates well with the mean Ti-O distance. The 25Mg and 113Cd NMR parameters are also reported for the relevant compounds.

Barium↗
PubMed1999

Cristobalite in volcanic ash of the soufriere hills volcano, montserrat, british west indies

Crystalline silica (mostly cristobalite) was produced by vapor-phase crystallization and devitrification in the andesite lava dome of the Soufriere Hills volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica, an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 percent) of crystalline silica. High levels of cristobalite in respirable ash raise concerns about adverse health effects of long-term human exposure to ash from lava dome eruptions.

Journal Article↗
PubMed1995

Magic-angle spinning nuclear magnetic resonance study of the structure of some PbO-Al2O3-P2O5 glasses.

Some PbO-Al2O3-P2O5 glasses have been investigated using 31P and 27Al magic-angle spinning nuclear magnetic resonance. The observed spectra could be divided into two groups. In the first, Al(IV), Al(V) and Al(VI) are observed, the 31P linewidths are large and the chemical shift range (-30 to -36 ppm) is typical of network phosphate. In the second group, only Al(VI) is observed, the 31P linewidths are much narrower and the shifts are less negative (-18 to -28 ppm).

Aluminum Oxide↗