EXAFS, XANES and 31P double-quantum MAS-NMR of (50− x/2)Na 2O– xBi 2O 3–(50− x/2)P 2O 5 glasses

Abstract

The structure of (50− x/2)Na 2O– xBi 2O 3–(50− x/2)P 2O 5 glasses was investigated by Bi L III XANES and EXAFS, 31P double-quantum MAS-NMR and 23Na MAS-NMR. From XANES and EXAFS analyses, Bi 3+ environment is found to be similar to that in the crystalline reference Na 3Bi(PO 4) 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 Bi 2O 3 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 Bi 2O 3. This structural description is in accordance with a mixed-network former effect of Bi 2O 3 and with the high polarizability of Bi 3+ that enables an asymmetric coordination and a progressive deformation of the oxygen shells when increasing the Bi 2O 3 content in the glass. This specific behavior explains the linear evolution of the glass transition temperature with x.