Polyamorphism and liquid-liquid phase transition in B2O3

Abstract

Pressure of 4 GPa applied on liquid ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ leads to the formation of fourfold coordinated boron atoms and the resulting pressure-quenched glasses reflect the morphology of a ``two-species'' liquid mainly formed from triangular $\mathrm{B}{\mathrm{O}}_{3}$ and tetrahedral $\mathrm{B}{\mathrm{O}}_{4}$ groups. Raman spectra of compacted glasses show that pressure quenching of the liquid preserves the two species, also favoring the formation of two superstructural units: boroxol rings $({\mathrm{B}}_{3}{\mathrm{O}}_{6})$ involving only $\mathrm{B}{\mathrm{O}}_{3}$ units and pentaborate groups (two boroxol rings linked by a fourfold coordinated boron atom). Calorimetric analysis up to the liquid state shows that these polyamorphic glasses are single-phase systems characterized by a single glass transition with a much higher ${T}_{g}$ and a lower thermodynamic fragility than those of normal v-${\mathrm{B}}_{2}{\mathrm{O}}_{3}$. Above ${T}_{g}$, a sharp endothermic process due to the inverse liquid-liquid phase transition converting the coordination of boron atoms from 4 to 3 is also revealed. It leads to recovering the classical structure (at ambient pressure) of the ``single-species'' liquid ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$.