Ionic transport of alkali in borosilicate glass. Role of alkali nature on glass structure and on ionic conductivity at the glassy state

• Ionic transport was investigated in borosilicate glass in connection with glass structural analysis.
• Alkali size increase induces a glass network expansion and a change of polymerization.
• Alkali size increase leads to a higher miscibility of silicate and borate units.
• Glass structure evolution with alkali size leads to a change in ionic transport properties.
• Activation energy of ionic conductivity and alkali jump distances were calculated.

Ionic transport of alkali was investigated in the SiO2–B2O3–R2O glass system (with R = Li, Na, K or Cs) by using impedance spectroscopy measurements at the glassy state. Transport mechanisms are also understood on the basis of glass structural analysis, such as X-ray total scattering and Raman spectroscopy, which helped to describe the alkali distribution and the network structure of borosilicate glasses.It was shown that an alkali size increase induces a glass network expansion, with weaker binding forces between alkali and non-bridging oxygens (NBO) and a change in network polymerization, with a Q3 unit increase and a “quasi-absence” of borosilicate unit in the case of lithium glass. The glass structure evolution with alkali size leads to a change in its macroscopic properties, especially glass transition temperature and electrical conductivity.The alkali conduction mechanism is described by an interstitial pair migration based on Frenkel defects in ionic crystals, which is thermally activated. Based on electrical conductivity data, activation energy of conductivity and alkali jump distances were calculated. The latter were linked to the values of bond lengths as determined by X-ray total scattering experiments, which pointed out a likely heterogeneous distribution of alkali in the borosilicate glasses studied.