Molecular dynamics investigation of lithium borate glasses: Local structure and ion dynamics

Abstract

The effect of alkali content and temperature on the microstructure of lithium borate glasses, $x{\mathrm{Li}}_{2}\mathrm{O}\ensuremath{-}(1\ensuremath{-}x){\mathrm{B}}_{2}{\mathrm{O}}_{3},$ has been investigated for glass compositions $x=0.2--0.5$ and temperatures up to 1250 K. The molecular dynamics technique has been applied, with Ewald summation and periodic boundary conditions, to a collection of ca. 256 particles confined in a primitive cubic cell and interacting through a Born-Mayer-Huggins-type potential augmented with three-body angular terms. The short-range order (SRO) structure was found to consist of boron-oxygen tetrahedral, ${\mathrm{B}\mathrm{\O{}}}_{4}^{\ensuremath{-}}$ $[\mathrm{\O{}}=\mathrm{bridging}$ oxygen atom (BO)], and triangular units with variable number of nonbridging oxygen (NBO) atoms, ${\mathrm{B}\mathrm{\O{}}}_{3},$ ${\mathrm{B}\mathrm{\O{}}}_{2}{\mathrm{O}}^{\ensuremath{-}},$ and ${\mathrm{B}\mathrm{\O{}}\mathrm{O}}_{2}^{2\ensuremath{-}}$ $({\mathrm{O}}^{\ensuremath{-}}=\mathrm{NBO}).$ The relative abundance of SRO units was determined and found to depend on both glass composition and temperature. Increasing ${\mathrm{Li}}_{2}\mathrm{O}$ content at constant temperature or increasing temperature at a fixed composition was shown to cause rearrangements of the SRO structure and to lead towards ${\mathrm{B}\mathrm{\O{}}\mathrm{O}}_{2}^{2\ensuremath{-}}$ units in the range of compositions and temperatures investigated. Such changes were expressed in terms of chemical equilibria involving the SRO units. The local environments hosting the Li ions were investigated and distinguished in two main types: the first type of site is formed by BO's, while the second type involves the participation of NBO's. The vibrational response of Li ions in the two types of site was computed and found to correlate very well with the experimental far-infrared profiles. Calculation of diffusion coefficients of Li ions showed that diffusion is carried out predominantly through NBO sites. In addition, glass regions rich in Li/NBO were found to develop with increasing lithium oxide content and to percolate eventually into microchannels suitable for ion migration.