Understanding the paradigm of vitrification-devitrification transitions in oxide network forming compounds using a revised positron lifetime analysis

Abstract

Vitrification and devitrification transitions in network oxide compounds such as silicates and borates keeping the same composition of crystalline and non-crystalline polymorphs are examined in terms of volumetric nanostructurization using a revised positron annihilation lifetime analysis. Assuming simple conversion between Ps-decay and positron-trapping channels responsible for three-component lifetime spectra, probability of polymorphous crystalline-vitreous transformations is estimated by κ parameter defined as relative changes in average lifetimes for direct (Ps-to-positron) and inverse (positron-to-Ps) trapping conversion. Both volume-expanded vitrification and volume-contracted devitrification processes are realized in SiO2-based network systems where disordered counterparts are fused quartz, amorphous films or cylindrical-porous glass (κ < ∼6%), while they are questionable in grown silica films (κ ∼ 8%). Polymorphous transformations are unfavorable in B2O3-based systems because of entropy gain rearranging superstructural grouping in their networks (κ∼28%). But in modified borates like lithium tetraborate Li2B4O7 where Ps- and positron-related traps are composed of Li vacancies, these transitions become possible (κ∼3.2 %).