Structural Study of α-Rhombohedral Boron at High Pressures

Abstract

Recently, it has been shown that, like β-rhombohedral boron (β-boron), α-rhombohedral boron (α-boron) exhibits superconductivity at high pressure. The transition pressure is similar to that of β-boron, that is, about 160 GPa. This paper presents a theoretical and experimental study of the structural change in α-boron at high pressures. High-pressure (up to 200 GPa) X-ray experiments were performed at a synchrotron-radiation facility. At ambient temperature, the crystal of α-boron is stable over the whole range of pressures examined. There is no phase transitions, such as to α-Ga type, which were previously predicted. All the structural parameters vary continuously with pressure. The cause of metallization is a significant contraction of the inter icosahedral three-center bond, which brings a higher coordination to the icosahedron and a transfer of charge from the intra icosahedoral bond to the three-center bond. Although everything is continuous, a careful analysis of the pressure dependence of the struct...