New B2O3 Crystals Predicted from Concurrent Molecular Dynamics Simulations and First-Principles Calculations

Molecular dynamics (MD) simulations, based on a new coordination-dependent charge-transfer potential, were used to study the behavior of crystalline B 2 O 3 in response to various thermal and mechanical constraints. This interaction potential allows for the charges on atoms to redistribute upon the formation and rupture of chemical bonds and dynamically adjusts to multiple coordination states for a given species. Our MD simulations predict that upon isotropic expansion and compression, the B 2 O 3 -I crystal transforms into new low- and high-density B 2 O 3 crystals, the stability of which we have further verified using first-principles calculations. The low-density B 2 O 3 crystals (B 2 O 3 -0) provide a key to understanding the anomalous thermomechanical behaviors of vitreous B 2 O 3 and the crystallization anomaly of this compound. The high-density B 2 O 3 crystal (B 2 O 3 -III), predicted from concurrent MD simulations and first-principles calculations, is different from the known high-pressure phase of B 2 O 3 -II crystal, even though the bonding is the same in these two phases. B 2 O 3 -III is characterized by a higher energy than B 2 O 3 -II at low pressures, but upon further compression the energies of these two phases become indistinguishable. The transformation from B 2 O 3 -I to B 2 O 3 -III appears to be kinetically favored, especially at low temperatures. Our studies indicate that the phase diagram of B 2 O 3 is much richer than previously known.