Polyamorphism and liquid-liquid phase transition in B2O3

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 $\mathrm{B}{\mathrm{O}}<sub>3</sub>$ and tetrahedral $\mathrm{B}{\mathrm{O}}<sub>4</sub>$ 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 $({\mathrm{B}}<sub>3</sub>{\mathrm{O}}<sub>6</sub>)$ involving only $\mathrm{B}{\mathrm{O}}<sub>3</sub>$ 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 ${T}<sub>g</sub>$ and a lower thermodynamic fragility than those of normal v-${\mathrm{B}}<sub>2</sub>{\mathrm{O}}<sub>3</sub>$. Above ${T}<sub>g</sub>$, 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 ${\mathrm{B}}₂{\mathrm{O}}₃$.