Nature of Structural Transformations in the B2O3 Glass under High Pressure

TL;DR

This study investigates the structural transformations of B2O3 glass under high pressure using X-ray diffraction, volumetric measurements, and first-principles simulations, revealing two broad, reversible transitions and predicting a super-dense phase at very high pressures.

Contribution

It provides detailed experimental and computational insights into the pressure-induced transformations and coordination changes in B2O3 glass, including the prediction of a new super-dense phase.

Findings

Two broad pressure-overlapping transitions identified.

The second transition is fully reversible.

Predicted formation of a super-dense phase with 5- and 6-fold coordinated B atoms.

Abstract

We report the results of the X-ray diffraction study of B2O3 glass in the pressure interval up to 10 GPa in the 300-700 K temperature range, the results of in-situ volumetric measurements of the glass at pressures up to 9 GPa at room temperature, and first-principles simulations data at pressures up to 250GPa. The behavior of B2O3 glass under pressure can be described as two broad pressure-overlapping transitions. The first transition starts at P > 1 GPa and proceeds without any changes in thecoordination number of the boron atoms; the other one starts at P > 5 GPa and is accompanied by a gradual increase of coordination number. The second transition is completely reversible; the residual densification of the B2O3 glass after decompression is associated with the incomplete reversibility of the first transformation. The fraction of boron atoms transferred to the 4-coordination state in the glass at P < 10 GPa is much smaller than was assumed from indirect experimental data [4], but is considerably larger (by tens of times) than was previously suggested by the classical molecular dynamics simulations [5,6]. The observed transformations under both compression and decompression are quite broad, contrary to the assumption in Ref. [7]. On the basis of ab-initio results, we also predict the one more transformation under higher pressures to a super-dense phase, in which B atoms become 5- and 6-fold coordinated.