# van der Waals forces stabilize low-energy polymorphism in B2O3: Implications for the crystallization anomaly

## TL;DR

This study uses advanced computational methods to show that van der Waals forces stabilize low-energy polymorphs of B$_2$O$_3$, explaining its tendency to form glassy structures instead of crystalline forms.

## Contribution

It reveals the crucial role of van der Waals interactions in stabilizing B$_2$O$_3$ polymorphs and explains the crystallization anomaly through first-principles calculations.

## Key findings

- Van der Waals forces stabilize the experimentally known B$_2$O$_3$ polymorph.
- Many metastable structures are close in energy to the glass.
- The best metastable polymorph shares motifs with the glass and a borosulfate compound.

## Abstract

The cohesive energies and structural properties of recently predicted, and never synthesized, B$_2$O$_3$ polymorphs are investigated from first principles using density functional theory and high-accuracy many-body methods, namely, the random phase approximation and quantum Monte Carlo. We demonstrate that the van der Waals forces play a key role in making the experimentally known polymorph (B$_2$O$_3$-I) the lowest in energy, with many competing metastable structures lying only a few kcal/mol above. Remarkably, all metastable crystals are comparable in energy and density to the glass, while having anisotropic and mechanically soft structures. Furthermore, the best metastable polymorph according to our stability criteria has a structural motif found in both the glass and a recently synthesized borosulfate compound. Our findings provide new perspectives for understanding the B$_2$O$_3$ anomalous behavior, namely, its propensity to vitrify in a glassy structure drastically different from the known crystal.