Towards guided metallic ions migration in network glass-formers: The Positronics approach in application to lithium tetraborate glass

Abstract

The Positronics approach grounded on positron annihilation lifetime spectroscopy treated in terms of x3-x2-CDA (coupling decomposition algorithm) is used to recognize nanostructurization-driven volumetric effects in lithium tetraborate glass g-Li2B4O7 under transition to crystalline state, doping by Ag and/or Gd activators and 2-h above-Tg annealing in air environment. Lithium vacancies and vacancy complexes are shown to be principal free-volume elements responsible for positron trapping and decaying of bound positron-electron (positronium, Ps) states in these glasses. Expected devitrification (glass-to-crystal) transition corresponds to direct (Ps-to-positron) trapping conversion in volume-contracted network due to transformation of Ps-hosting sites in positron traps.Plausible migration scenarios for metallic activators in g-Li2B4O7 include occupation of vacancy-type positron traps and Ps-decay holes by Ag+ and/or Gd3+ ions, totally reducing positron-trapping and Ps-decaying contributions. Effect of thermal annealing in metal-doped lithium tetraborate glass is not related to simple de-occupation of vacancy-type defects due to escape of Ag + or Gd3+ ions. Annealing-stimulated transformations are guided by relaxation towards defect-free glass structure, being disturbed by competitive defect formation and destruction processes accelerated by interaction with oxygen. Since non-elementary nature of trapping-modification processes in doped glasses subjected to annealing, the formalism of x3-x2-CDA cannot be reasonably applicable.