Molecular dynamics investigation of mixed-alkali borate glasses: Short-range order structure and alkali-ion environments

Abstract

Structural properties of mixed-alkali borate glasses, $0.3[(1\ensuremath{-}x){\text{Li}}_{2}\text{O-}x{\text{Cs}}_{2}\text{O}]\ensuremath{-}0.7{\text{B}}_{2}{\text{O}}_{3}$ and $0.3[(1\ensuremath{-}x){\text{Li}}_{2}\text{O-}x{\text{Na}}_{2}\text{O}]\ensuremath{-}0.7{\text{B}}_{2}{\text{O}}_{3}$, have been studied by molecular dynamics simulations at $T=300\text{ }\text{K}$ and for several values of the alkali mixing parameter, $x$, to explore structural foundations of the mixed-alkali effect (MAE). The short-range order (SRO) structure was found to consist of borate tetrahedra, $\text{B}{\ensuremath{\emptyset}}_{4}^{\ensuremath{-}}$, and of neutral, $\text{B}{\ensuremath{\emptyset}}_{3}$, and charged, $\text{B}{\ensuremath{\emptyset}}_{2}{\text{O}}^{\ensuremath{-}}$, triangular units [$\ensuremath{\emptyset}=\text{bridging}$ oxygen atom]. The abundance of $\text{B}{\ensuremath{\emptyset}}_{4}^{\ensuremath{-}}$ units was found to decrease from Li to Cs and to exhibit negative deviation from linearity in Li-Cs glasses. However, no appreciable change in SRO structure was detected in mixed Li-Na glasses. Even though alkali metal $(M)$ ions occupy in mixed glasses sites, i.e., coordination environments with O atoms, similar to those formed in single alkali borates, mixing was found to affect the $M\text{-O}$ bonding properties of dissimilar alkalis in an opposite manner. Thus, for both systems investigated here the Li ion-coordination environment was found to become better defined and the Li-O interactions to strengthen upon alkali mixing, whereas the Cs-O and Na-O interactions become progressively weakened. The origin of these trends was traced to cationic environments formed around nonbridging oxygen (NBO) atoms in glass; it was found that the dominant cation configurations around NBOs consist of dissimilar cations in mixed-alkali glasses. The formation of dissimilar ion pairs affects by polarization effects the bonding and vibrational properties of metal ions in their oxide sites. This was demonstrated for Li-Cs glasses by both experimental and calculated far infrared spectra, where the metal ion-oxide site vibrations are strongly active. It was discussed that the preference of unlike-alkali ion pairing around NBOs and the consequent drastic reduction in the number of NBOs that sense like-cations could provide a structural explanation for the MAE.