Site discrimination in the crystalline borophosphate Na5B2P3O13 using advanced solid-state NMR techniques.

A comprehensive solid-state NMR investigation on crystalline Na(5)B(2)P(3)O(13) is presented. Triple-quantum magic angle spinning (TQMAS) and rotational echo double resonance (REDOR) studies are used for accurate determinations of the (11)B, (23)Na and (31)P interaction parameters. Based on these results and complementary quantum mechanical calculations, plausible site assignments can be derived. Generally, the results show that detailed, quantitative information about structures in borophosphate compounds can be obtained by investigating both the local site environments characterized by chemical shift and quadrupolar interaction parameters and the correlated dipolar interactions to atoms in the second coordination sphere.

Structural studies of NaPO3-MoO3 glasses by solid-state nuclear magnetic resonance and Raman spectroscopy.

Vitreous samples were prepared in the (100 - x)% NaPO(3)-x% MoO(3) (0

Medium-range order in sodium phosphate glasses: a quantitative rotational echo double resonance solid state NMR study.

Sodium ultraphosphate glasses (Na(2)O)(x)(P(2)O(5))(1-x) show a strongly non-linear dependence of the glass transition temperatures T(g)(x) on composition. To explore the structural origins of this behaviour, local and medium range ordering processes have been investigated by state-of-the-art (23)Na high-resolution and dipolar NMR spectroscopies. In particular, (31)P(23)Na) and (23)Na((31)P) rotational echo double resonance (REDOR) experiments have been analyzed to yield quantitative constraints for the structural description of these glasses. The sodium ions are found to be randomly distributed and, for x < 0.25, spatially correlated with a single metaphosphate-type Q((2)) unit at a distance of 330 pm. In this region, unusual compositional trends observed for the (23)Na chemical shifts and nuclear electric quadrupolar coupling constants, measured by triple-quantum magic-angle spinning (TQMAS) NMR, suggest a systematic decrease of Na coordination number with x. At higher sodium contents (x > 0.25), the magnitude of the (31)P((23)Na) dipolar interaction increases markedly, indicating a significantly increased extent of Q((2))-Na-Q((2)) crosslinking. Based on these results, a comprehensive description of medium-range order in sodium ultraphosphate glasses is developed, suggesting that the T(g)(x) dependence is closely linked to changes in the relative phosphorus/sodium distance distributions.

Dipolar solid state NMR approaches towards medium-range structure in oxide glasses.

Modern solid state nuclear magnetic resonance presents new powerful opportunities for the elucidation of medium range order in glasses in the sub-nanometer region. In contrast to standard chemical shift spectroscopy, the strategy presented here is based on the precise measurement and quantitative analysis of internuclear magnetic dipole-dipole interactions, which can be related to distance information in a straightforward manner. The review discusses the most commonly employed experimental techniques, producing dipolar coupling information in both homo- and heteronuclear spin systems. The approach is particularly powerful in combination with magic-angle sample spinning, producing site-resolved dipolar coupling information. We present new applications to oxide-based network glasses, permitting network connectivities and spatial cation distributions to be elucidated.

Cation environments and spatial distribution in Na2O-B2O3 glasses: new results from solid state NMR.

The spatial distribution of the sodium ions in sodium borate glasses with composition (Na2O)x(B2O3)1+x (0 < or = x < or = 0.30) has been studied by complementary high resolution and dipolar solid state NMR experiments. 23Na-23Na homonuclear dipole-dipole couplings measured via 23Na spin echo decay spectroscopy indicate the absence of cation clustering at all compositions. Consistent with this result, 11B{23Na} rotational echo double resonance (REDOR) measurements indicate that the trigonal BO3/2 groups and the four-coordinate BO4/2(-) units experience 23Na local dipolar fields of similar magnitudes. Both experiments suggest a sodium spatial distribution that is essentially statistical. The dipolar coupling data are modelled consistently on the basis of a cubic NaCl-type lattice, from which an appropriate number of sodium ions have been randomly removed to reproduce the Na+ number density of the glass under consideration. In addition, at very low Na+ concentrations (x < 0.12) a compositionally independent local sodium environment is formed, which is characterized by a significant Na+ BO4/2(-) pair correlation involving an internuclear distance of 316 pm. For higher sodium oxide contents (x > 0.20) the 23Na-23Na dipole-dipole couplings suggest a gradual transition from a random towards a more homogeneous sodium distribution.

Site connectivities in silver borophosphate glasses: new results from 11B{31P} and 31P{11B} rotational echo double resonance NMR spectroscopy.

Local and medium range order in the glass system 50Ag2O-50[(B2O3)x-(P2O5)(1-x)] (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) have been investigated by high-resolution solid state nuclear magnetic resonance (NMR) techniques. The detailed local site distribution has been derived from deconvolution analysis of the 11B and 31P magic-angle spinning (MAS) NMR signals. Quantitative information regarding the extent of boron-oxygen-phosphorus connectivity has been obtained on the basis of 11B[31P} and 31P{11B} rotational echo double resonance experiments. Incorporation of borate into silver metaphosphate glasses produces four-coordinate BO4/2- sites, which crosslink the metaphosphate chains, resulting in a significant increase in the glass transition temperature. Furthermore, the presence of borate favors the disproportionation of P(2) chain-like units into P(1) and P(3) sites, an effect not observed in binary alkali phosphate glasses. Finally, borate incorporation beyond x=0.3 results in the formation of neutral BO3/2 units, indicating some net charge transfer from the borate to the phosphate network former species. This latter result corresponds to the general metal ion scavenging effect observed for phosphate species in other mixed network former glass systems. In the present system, the effect is relatively moderate, however, suggesting that anionic BO4/2- groups are stabilized by the interaction with the phosphate groups.