Structure−Property Relations in Mixed-Network Glasses: Multinuclear Solid State NMR Investigations of the System xAl2O3:(30 − x)P2O5:70SiO2

Abstract

New mixed-network glasses along the composition line xAl2O3−(30 − x)P2O5−70SiO2 have been prepared and characterized in terms of their density, thermal expansion coefficient, refractive index, and characteristic temperatures. The compositional changes in these macroscopic properties have been correlated with structural information, obtained via Raman spectroscopy and state-of-the-art solid state NMR techniques, including 27Al, 29Si, and 31P magic-angle spinning (MAS) NMR, 27Al triple quantum MAS NMR, as well as static 31P spin echo decay spectroscopy. In addition, the extent of P−O−Al connectivity has been quantified on the basis of 27Al{31P} rotational echo double resonance (REDOR) and 31P{27Al} rotational echo adiabatic passage double resonance (REAPDOR) measurements. Both the macroscopic and the structural properties show nonlinear dependences on x, including abrupt changes at a nominal Al/P ratio of 1 (x = 0.15), where no glasses can be formed by melt quenching under the conditions used in this study. The structure of phosphorus-rich glasses (Al/P < 1) is characterized by four-, five- and six-coordinated Al species, whose second coordination sphere is dominated by phosphorus. 31P static and MAS NMR spectra suggest the presence of at least three distinct phosphorus environments, corresponding to silicon-bonded P(3) units, anionic metaphosphate P(2) species interacting with octahedral aluminum, and tetrahedral PO4/2 groups (P(4) units) bonded similarly as in AlPO4. (In this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) The latter species is also the dominant phosphorus environment in the Al-rich glasses (Al/P > 1), where the alumina component is involved in Al−O−P, Al−O−Si, and possibly also Al−O−Al linkages. All of these results indicate that the structure of these glasses is dominated by the strong mutual affinity of the phosphorus oxide and alumina components. To quantify this affinity, the experimental REDOR and REAPDOR results have been compared with a cluster model assuming that both components react completely under formation of aluminum phosphate-like domains, thereby maximizing the number of Al−O−P linkages. Both the REDOR and the REAPDOR results show, however, clear deviations from such a structural scenario, supporting a more homogeneous glass structure with a certain degree of connectivity randomization.