Crystal Stabilized by Defects : Elemental Boron

Abstract

β-Rhombohedral boron is the thermodynamically stable allotrope of boron at ambient pressure and at near the melting temperature. X-ray diffraction analysis has shown that this crystal has 423 atomic positions in its hexagonal unit cell, and only 320 of them are occupied. More precisely, 23 out of 320 atoms are randomly located over the 126 atomic sites. Recently, a theoretical study has shown that the partial occupancy is necessary to form a closed shell electronic structure. Moreover, it was also shown that there exists a competition between the interaction and the lattice structure of partial occupancy leading to realization of geometrical frustration in elemental solid. In this article, we will discuss on the historical background of boron research, on the recent progress in understanding its unique property, and on its relation to the third law of thermodynamics.