Topological Modification and Elastic Softening of Sodium Borophosphate Glasses Through Substitution of Oxygen by Chlorine

Ionic glasses exhibit peculiar chemical formulations, atomic structures, and macroscopic properties. In particular, structure–property relationships cannot be explained using the classical network hypothesis; a standing challenge for understanding variations in ion mobility, vibrational dynamics and non-linear mixing effects. Here, we report on the substitution of oxygen by chlorine atoms in [(Na2O)0.4-x/2(NaCl)x](B2O3)0.48(P2O5)0.12 glasses up to a substitution degree of around 10 %, effectively combining the enhanced electrical conductivity of halides and the chemical durability of oxide glasses. A mechanism of chlorine introduction, specific to borophosphate glasses, is proposed. The substitution of O2– ions for Cl– ions induces polymerization of the borophosphate network and conversion of BO4 groups into BO3 groups, thereby forming Na+ and Cl– enriched regions dispersed within a sodium-poor borophosphate network. Then, the associated increase in electrical conductivity and the dramatic changes in the elastic constants can be explained within the mixed random network model.