Structure-property relationships of alkaline earth borosilicate glasses were determined using solid state NMR, the glass transition temperature and density. A sharing of alkaline earth oxide amongst the borate and silicate networks teas observed and modelled. Density and T-g data were compared with the constructed models providing support to structural ideas. Comparisons to structure-property relations of alkali borosilicate glasses were also made.

Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na8Si46 and Na2Ba6Si46. Although their structure and properties are well described, there is little understanding of the formation mechanism. Na8Si46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si-4(4-) polyanions.,In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25-500 degrees C at 8 degrees C/min) under vacuum conditions (10(-4) bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state Na-23 NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na8Si46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the dathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.

There is increasing evidence that amorphous inorganic materials play a key role in biomineralisation in many organisms, however the inherent instability of synthetic analogues in the absence of the complex in vivo matrix limits their study and clinical exploitation. To address this, we report here an approach that enhances long-term stability to >1 year of biologically relevant amorphous metal phosphates, in the absence of any complex stabilisers, by utilising pyrophosphates (P2O74-); species themselves ubiquitous in vivo. Ambient temperature precipitation reactions were employed to synthesise amorphous Ca2P2O7 center dot nH(2)O and Sr2P2O7 center dot nH(2)O (3.8 < n < 4.2) and their stability and structure were investigated. Pair distribution functions (PDF) derived from synchrotron X-ray data indicated a lack of structural order beyond similar to 8 angstrom in both phases, with this local order found to resemble crystalline analogues. Further studies, including H-1 and P-31 solid state NMR, suggest the unusually high stability of these purely inorganic amorphous phases is partly due to disorder in the P-O-P bond angles within the P2O7 units, which impede crystallization, and to water molecules, which are involved in H-bonds of various strengths within the structures and hamper the formation of an ordered network. In situ high temperature powder X-ray diffraction data indicated that the amorphous nature of both phases surprisingly persisted to similar to 450 degrees C. Further NMR and TGA studies found that above ambient temperature some water molecules reacted with P2O7 anions, leading to the hydrolysis of some P-O-P linkages and the formation of HPO42- anions within the amorphous matrix. The latter anions then recombined into P2O7 ions at higher temperatures prior to crystallization. Together, these findings provide important new materials with unexplored potential for enzyme-assisted resorption and establish factors crucial to isolate further stable amorphous inorganic materials.

Tin phosphate glasses, of general formula x SnO . (1 - x)P2O5 (0.3 < x < 0.8), have been prepared by conventional melt-quench techniques and their structures studied using P-31 and Sn-119 nuclear magnetic resonance. The distribution of [PO4] Q(n) species changes with composition in accordance with the simple binary model, and the changes in chemical shift can be explained by the redistribution of electron charge from the P=O double bond. Sn(II) is found to occupy a highly asymmetric site, typical of a sterically active lone pair of electrons. The Sn-119 parameters of the chemical shift tensor change systematically with x, reflecting the change in local environment from one where the next nearest neighbours are predominantly Q(2) phosphorus to one where they are predominantly Q(0) phosphorus.

119Sn and 17O magic angle spinning NMR spectra of xTiO2 · (1 − x)SnO2 (x = 0.75, 0.8, 0.85) gels are presented. In these compounds a range of tin environments are observed, consistent with the cations being randomly distributed. In the initial gels, the oxygen environment is already rutile-like and no resonances from pure SnO2 are observed.

The structures of a series of hydrous albite glasses quenched from melts at high pressures and temperatures have been studied using 29 Si, 23 Na, 27 Al, and 1 H nuclear magnetic resonance. Changes in the isotropic chemical shift, the chemical shift dispersion, and the mean nuclear quadrupole coupling constant for 23 Na as a function of dissolved water concentration were deduced from spectra obtained at two different magnetic fields. Major changes in the sodium environment occur, but the spectra for 29 Si and 27 Al, and hence their structural environments, are similar throughout the range of water concentrations studied (0–67 mol%). No previous model is consistent with the results of this study. The data suggest the existence of the following structural features: i) exchange of H + for Na + as a charge-balancing cation; ii) formation of Na(OH) complexes; iii) incorporation of molecular water; iv) no octahedrally coordinated aluminium; v) no Al-OH or Si-OH. These features can be summarised in terms of the equilibrium NaAlSi 3 O 8 + H 2 O ⇋ HAlSi 3 O 8 + Na ( OH ). In contrast to all previous interpretations, we see no evidence for depolymerisation of the aluminosilicate framework, although an increase in the symmetry of the aluminium environments and decrease in the chemical shift dispersion of the sodium environments suggests a more ‘ordered’ structure than in the dry glass. If the structures of hydrous albite melts are the same as those of the glasses studied here the current understanding of the effect of dissolved water on the physical properties of felsic melts must be reassessed.