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 Q²_m 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 AlO_n 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 Q²_m 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.