Pressure-induced structural transformations in glass0.3Li2O-0.7B2O3: A molecular dynamics study

Molecular dynamics (MD) simulations were used to study structural transformations in glass $0.3{\text{Li}}_{2}\text{O-}0.7{\text{B}}_{2}{\text{O}}_{3}$ induced by high pressure up to 300 GPa. Pressure-induced coordination changes were found to involve the gradual transformations of boron $^{[3]}\text{B}\ensuremath{\rightarrow}^{[4]}\text{B}\ensuremath{\rightarrow}^{[5]}\text{B}\ensuremath{\rightarrow}^{[6]}\text{B}$ and the transformations $^{[2]}\text{O}\ensuremath{\rightarrow}^{[3]}\text{O}\ensuremath{\rightarrow}^{[4]}\text{O}$ of oxygen, where superscripts in brackets indicate coordination numbers. Four different pressure zones were distinguished by probing the relative population of boron and oxygen atoms in different coordination states, and found to correspond to pressures below 5 GPa (I), 5--15 GPa (II), 15--80 GPa (III), and 80--300 GPa (IV). The results of MD simulations are in good agreement with reported findings by inelastic x-ray scattering for pressures up to 30 GPa attainable currently by experiment. More detailed structural information was obtained by MD for zone III where the boron and oxygen coordination numbers were found constant at $\text{B}[\text{O}]=4$ and $\text{O}[\text{B}]\ensuremath{\approx}2.35$ in a broad pressure range. It was shown that the structure of glass in this zone is consistent with the engagement of 91.5% of four-coordinated boron atoms, $^{[4]}\text{B}$, in tricluster arrangements where three $^{[4]}\text{B}$ atoms are corner sharing the same three-coordinated oxygen atom, $^{[3]}\text{O}$, and have the overall stoichiomertry of ${[{^{[4]}\text{B}}_{3}\text{ }{^{[3]}\text{O}}_{2}\text{ }{^{[2]}\text{O}}_{3}]}^{\ensuremath{-}1}$. While this tricluster is the dominant structural element, the remaining 8.5% of $^{[4]}\text{B}$ atoms are found in tetrahedral borate units which involve nonbridging oxygen atoms (approximately 4.3%) as well as $^{[3]}\text{O}$ and $^{[2]}\text{O}$ oxygen atoms.