MAS-NMR has been used to explore the structure of sodium phospho-alumino-silicate glasses. Adding P 2 O 5 to sodium silicates causes repolymerisation of the silicate network and formation of sodium ortho-, pyro- and eventually metaphosphate units. At high P 2 O 5 concentrations the structure is determined by the incorporation of silica into a sodium phosphate glass as either chain end species or as an “SiP 2 O 7 ” phase. Removal of sodium from these glasses or substitution of alumina for P 2 O 5 destroys the “SiP 2 O 7 ” phase.

Magic angle spinning NMR of alkali phospho-alumino-silicate glasses

Structure of ZrV2O7from −263 to 470°C

Interatomic Distance Predictions for Computer Simulation of Crystal Structures

A routinely used assumption when interpreting two-dimensional NMR spectra obtained with a commonly used double-quantum (DQ) magic-angle-spining (MAS) pulse sequence referred to as the refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) [A. Lesage, M. Bardet, and L. Emsley, J. Am. Chem. Soc. 121, 10987 (1999)] has been that correlation peaks are only observed for pairs of nuclei with a through-bond connectivity. The validity of this assumption is addressed here by theory, experiment, and computer simulations. If the isotropic chemical shifts of the two nuclei are different and the MAS frequency is far from rotational resonance, the theoretical description demonstrates that DQ correlation peaks are indeed indicative of a J coupling. However, if the isotropic chemical shifts are the same, it is shown that DQ peaks can appear for pairs of nuclei even in the absence of a through-bond J coupling. These peaks appear in the specific case of a pair of nuclei with a nonzero throug...

Through-space contributions to two-dimensional double-quantum J correlation NMR spectra of magic-angle-spinning solids

The structure of cubic ZrP2O7 at room temperature has been solved and refined using a combination of modeling and high-resolution neutron powder diffraction data. The cell edge is 24.74 A, the space group is Pa3, and Z is 108. For those P2O7 units not on a 3-fold axis, the P−O−P angles range from 134° to 162°. Two crystallographically distinct P2O7 groups are on three fold axes with P−O−P angles thus constrained to be 180° on average. The structure of cubic ZrP2O7 was also refined from data taken at 227, 290, 371, 435, and 610 °C. The 3 × 3 × 3 superstructure present at room temperature disappears at about 290 °C, and all P−O−P angles of P2O7 are then constrained by symmetry to be 180° on average. The exceptionally low thermal expansion shown by ZrP2O7 above 290 °C is likely related to the unusual P−O−P angle.

W. Gebert

Papers

Computer simulation of crystal structures applied to the solution of the superstructure of cubic silicondiphosphate