Structure of Ternary Aluminum Metaphosphate Glasses

Ternary metaphosphate glasses in the systems (1 - x)NaPO3 center dot xAl(PO3)(3), (1-x)KPO3 center dot xAl(PO3)(3), and (1 - x)Pb(PO3)(2) center dot xAl(PO3)(3)(0 <= x <= 1) were analyzed using a set of P-31, Al-27, and Na-23 high-resolution NMR and X-ray photoelectron spectroscopy techniques, to determine the phosphate speciation and the short- and medium-range order properties of the P and Al connectivity. O-1s XPS data confirm that the number of oxygen atoms linking two phosphorus remains close to 33% for all of the glasses, consistent with the exclusive presence of Q(2) units, in agreement with P-31 MAS NMR data. The increasing formation of Al-O-P linkages With increasing x is indicated by a new O-1s peak, with binding energies near 532 eV, and systematic changes in the P-31 MAS NMR chemical shifts. The presence of Q(m)(2) phosphate groups (m being the number of P-O-Al bonds per tetrahedron, 0 <= m <= 2) Was analyzed by P-31 MAS NMR and P-31{Al-27} REAPDOR experiments The REDOR technique was applied to the heteronuclear spin systems P-31/Al-27 and P-31/Na-23, to analyze the local structure around these species. Al-27 MAS NMR and Al-27 triple quantum MAS were:applied respectively to determine the coordination state of Al and the values of isotropic chemical shift and electric quadrupole coupling parameters of Al-27. The results points to common features in the glass structure of these ternary phosphates. The most probable environment of Al has six dose P atoms, with no evidence of Al-O-Al bonds, showing that the connection between AlOn and PO4 is attained through corners. There is no evidence of local segregation of cationic species in the phosphate matrix A definite precedence in the formation of Q(m)(2) units was found as the Al concentration is increased, consisting of the progressive conversion of Q(0)(2) to Q(1)(2) and then Q(1)(2) to Q(2)(2) units with increasing x. Up to intermediate values of x, the speciation shows an above-random trend compatible with a binary distribution {Q(0)(2),Q(1)(2)) for the K-Al and Pb-Al systems.