Structure of sodium phosphate glasses containing Al2O3 and/or Fe2O3. Part I

The relationship between the composition and structure for three series of glasses: Na2O–Al2O3–P2O5 (NAP); Na2O–Al2O3–Fe2O3–P2O5 (NAFP), and Na2O–Fe2O3–P2O5 (NFP) has been studied. The structural changes in the NAP, NAFP and NFP glasses have been investigated by Raman and by Raman difference spectroscopy (RDS). The Raman spectra show that the addition of Al2O3 content to sodium phosphate glasses has a different effect on the phosphate network than the addition of Fe2O3. With increasing Fe2O3 content up to 20 mol% the structure changes from the chain-like metaphosphate to the pyrophosphate structure. The iron ions play an important role in forming P–O–Fe bonds that strengthen the cross-bonding of shorter pyrophosphate chains. On the other hand, with increasing Al2O3 content up to 20 mol%, the sodium metaphosphate is replaced by aluminium metaphosphate where Al(OP)6 units cross-link phosphate chains. The addition of Fe2O3 to Al2O3 in phosphate glasses containing both Al2O3 and Fe2O3 (NAFP series), enhances the formation of pyrophosphate units because iron ions have stronger effect on the depolymerization of metaphosphate chains if compared to the aluminium ions.