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.

Automated summary

Applying high pressure on liquid B2O3 results in the formation of fourfold coordinated boron atoms and the presence of two species in the liquid structure. Pressure quenching of the liquid preserves the two species, forming boroxol rings and pentaborate groups. These polyamorphic glasses have a single glass transition temperature, higher T-g and lower thermodynamic fragility compared to normal B2O3. An inverse liquid-liquid phase transition is observed at higher temperatures.