Structural Study of α-Rhombohedral Boron at High Pressures

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 structural parameters by comparing experiment with ab initio calculation, reveals a characteristic change in the angle of the rhombohedral lattice and some of the internal coordinates of atoms at about 50 GPa. This is the pressure at which the metallization process begins. This structural change can be observed as the step in the pressure dependence of the electrical resistivity and the phonon softening. The interrelationships underlying these changes have been established on theoretical grounds. The long-debated issue of whether the inter icosahedral bond is stronger than intra icosahedral bond has been resolved: the traditional understanding that the former is stronger has been proven. A direct piece of evidence for this is the initial deviation of the inter icosahedral bond from the lattice vector.