Structure of Ternary Aluminum Metaphosphate Glasses

Ternary metaphosphate glasses in the systems (1 – x)NaPO3·xAl(PO3)3, (1 – x)KPO3·xAl(PO3)3, and (1 – x)Pb(PO3)2·xAl(PO3)3 (0 ≤ x ≤ 1) were analyzed using a set of 31P, 27Al, and 23Na 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 Q2 units, in agreement with 31P 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 31P MAS NMR chemical shifts. The presence of Q2m phosphate groups (m being the number of P–O–Al bonds per tetrahedron, 0 ≤ m ≤ 2) was analyzed by 31P MAS NMR and 31P{27Al} REAPDOR experiments. The REDOR technique was applied to the heteronuclear spin systems 31P/27Al and 31P/23Na, to analyze the local structure around these species. 27Al MAS NMR and 27Al 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 27Al. The results points to common features in the glass structure of these ternary phosphates. The most probable environment of Al has six close 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 Q2m units was found as the Al concentration is increased, consisting of the progressive conversion of Q20 to Q21 and then Q21 to Q22 units with increasing x. Up to intermediate values of x, the speciation shows an above-random trend compatible with a binary distribution {Q20,Q21} for the K–Al and Pb–Al systems.