Short-range structure and cation bonding in calcium-aluminum metaphosphate glasses.

Comprehension of short- and medium-range order of phosphate glasses is a topic of interest, due to the close relation between network structure and mechanical, thermal, and optical properties. In this work, the short-range structure of glasses (1 - x)Ca(PO 3 ) 2 .xAl(PO 3 ) 3 with 0 ≤ x ≤ 0.47 was studied using solid-state nuclear magnetic resonance spectroscopy, Raman spectroscopy, density measurements, and differential scanning calorimetry. The bonding between a network modifier species, Al, and the network forming phosphate groups was probed using high-resolution nuclear magnetic resonance spectroscopy of 2 7 Al and 3 1 P. Changes in the compositional behavior of the density, glass transition temperature, PO 2 symmetric vibrations, and Al coordination number were verified at around x = 0.30. 3 1 P NMR spectra show the presence of phosphorus in Q 2 sites with nonbridging oxygens (NBOs) coordinated by Ca ions and also Q 2 sites with one NBO coordinated by Al (namely, Q 2 (1Al)). The changes in the properties as a function of x can be understood by considering the mean coordination number measured for Al and the formation of only Q 2 and Q 2 (1Al) species. It is possible to calculate that a network formed only by Q 2 (1Al) phosphates can just exist up to the upper limit of x = 0.48. Above this value, Q 2 (2Al) species should appear, imposing a major reorganization of the network. Above x = 0.30 the network undergoes a progressive reorganization to incorporate Al ions, maintaining the condition that only Q 2 (1Al) species are formed. These observations support the idea that bonding principles for cationic species inferred originally in binary phosphate glasses can also be extended to ternary systems.