Polyamorphism and liquid-liquid phase transition in B2O3

Pressure of 4 GPa applied on liquid B2O3 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 BO3 and tetrahedral BO4 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 (B3O6) involving only BO3 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-B2O3. 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 B2O3.