Determination of Phase Stability of Elemental Boron

Boron is an important element, used in applications from superhard materials to superconductors. Boron exists in several forms (allotropes) and, surprisingly, it was not known which form (alpha or beta) is stable at ambient conditions. Through experiment, we quantify the relative stability of alpha-boron and beta-boron as a function of temperature. The ground-state energies of alpha-boron and beta-boron are nearly identical. For all temperatures up to 2000 K, the complicated beta-boron structure is more stable than the simpler alpha-boron structure at ambient pressure. Below 1000 K, beta-boron is entropically stabilized with respect to alpha-boron owing to its partially occupied sites, whereas at higher temperatures beta-boron is enthalpically stabilized with respect to alpha-boron. We show that alpha-boron only becomes stable on application of pressure.