EXAFS, XANES and 31P double-quantum MAS-NMR of (50−x/2)Na2O–xBi2O3–(50−x/2)P2O5 glasses Original Research Article

The structure of (50−x/2)Na2O–xBi2O3–(50−x/2)P2O5 glasses was investigated by Bi LIII XANES and EXAFS, 31P double-quantum MAS-NMR and 23Na MAS-NMR. From XANES and EXAFS analyses, Bi3+ environment is found to be similar to that in the crystalline reference Na3Bi(PO4)2, with two distinct oxygen shells. The first shell contains 5.5±0.5 oxygens and EXAFS data are best fitted with a two sub-shell model. The Bi–O distances in this first asymmetric shell do not change with the increase in Bi2O3 content in the glasses, but a qualitative evolution of the second oxygen shell is observed. The sodium environment is not affected by the introduction of bismuth in the glass network. The metaphosphate glass network undergoes gradual and systematic depolymerization by Bi2O3. This structural description is in accordance with a mixed-network former effect of Bi2O3 and with the high polarizability of Bi3+ that enables an asymmetric coordination and a progressive deformation of the oxygen shells when increasing the Bi2O3 content in the glass. This specific behavior explains the linear evolution of the glass transition temperature with x.