Quantum and classical dynamics of intrinsic defects and equation of state of elemental boron*

Among of third group elements, boron exhibits relatively strong covalent character with trivalency leading to the strong tendency of forming icosahedra based building units: the property that leads to the complex allotropy. In case of beta-rhombohedral boron, it has been known that the basic skeleton deviates largely from the close shell electronic structure, and reconciled by large number of interstitials and vacancies, which forms quasi two-dimensional structure that can be mapped on to a frustrated Ising model. In this presentation, we will discuss the dynamical properties of these defect states at low and high temperatures. At low temperature, we show that tunneling splitting level of some interstitials are in a measurable energy scale owing partly to the close proximity of interstitial positions, while at high temperature, significant number of these defects become mobile where the skeletal atoms largely stay at the original crystallographic positions; the state that resembles to superionic solids. Finally, we will discuss about the inconsistency in the reported equation of states near the melting point.