Structure of Ternary Aluminum Metaphosphate Glasses

Ternary metaphosphate glasses in the systems (1<i> – x</i>)NaPO<sub>3</sub>·<i>x</i>Al(PO<sub>3</sub>)<sub>3</sub>, (1 – <i>x</i>)KPO<sub>3</sub>·<i>x</i>Al(PO<sub>3</sub>)<sub>3</sub>, and (1 – <i>x</i>)Pb(PO<sub>3</sub>)<sub>2</sub>·<i>x</i>Al(PO<sub>3</sub>)<sub>3</sub> (0 ≤ <i>x</i> ≤ 1) were analyzed using a set of <sup>31</sup>P, <sup>27</sup>Al, and <sup>23</sup>Na 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<sup>2</sup> units, in agreement with <sup>31</sup>P MAS NMR data. The increasing formation of Al–O–P linkages with increasing <i>x</i> is indicated by a new O-1s peak, with binding energies near 532 eV, and systematic changes in the <sup>31</sup>P MAS NMR chemical shifts. The presence of Q<sup>2</sup><sub><i>m</i></sub> phosphate groups (<i>m</i> being the number of P–O–Al bonds per tetrahedron, 0 ≤ <i>m</i> ≤ 2) was analyzed by <sup>31</sup>P MAS NMR and <sup>31</sup>P{<sup>27</sup>Al} REAPDOR experiments. The REDOR technique was applied to the heteronuclear spin systems <sup>31</sup>P/<sup>27</sup>Al and <sup>31</sup>P/<sup>23</sup>Na, to analyze the local structure around these species. <sup>27</sup>Al MAS NMR and <sup>27</sup>Al 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 <sup>27</sup>Al. 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<sub><i>n</i></sub> and PO<sub>4</sub> 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<sup>2</sup><sub><i>m</i></sub> units was found as the Al concentration is increased, consisting of the progressive conversion of Q<sup>2</sup><sub>0</sub> to Q<sup>2</sup><sub>1</sub> and then Q<sup>2</sup><sub>1</sub> to Q<sup>2</sup><sub>2</sub> units with increasing <i>x</i>. Up to intermediate values of <i>x</i>, the speciation shows an above-random trend compatible with a binary distribution {Q<sup>2</sup><sub>0</sub>,Q<sup>2</sup><sub>1</sub>} for the K–Al and Pb–Al systems.