Eiji Nishibori
University of Tsukuba, Japan
Title: Observation of π electron in metal hexaboride through X-ray charge density
Biography
Biography: Eiji Nishibori
Abstract
Metal hexaborides MB6, where M is an alkaline earth or rare earth metal exhibit metallic and semiconductor properties by changing the M ion. The metal hexaborides have a B6 octahedron in the structure. The M ion is located at the body-center surrounded by the B6 octahedra. The B6 octahedron has 18 valence electrons. Two electrons per B6 octahedron are required to fulfill the bonding orbital of B6. Metal hexaborides with divalent metal ions are considered to be semiconductor and with the trivalent ions are metal from the consideration. Theoretical study suggests that the p-electron like an anti-bonding orbital of B6 contributes electrical conductivity in the metallic trivalent MB6. We investigated the charge densities of divalent and trivalent metal hexaborides, semiconducting BaB6 and metallic LaB6 using the d>0.22 Å ultra-high resolution synchrotron radiation X-ray diffraction data by a multipole refinement and a maximum entropy method. High resolution powder diffraction data were measured at SPring-8. The strong inter-octahedral and relatively weak intra-octahedral boron-boron bonds were observed in the charge densities. A difference of valence charge densities between LaB6 and BaB6 was calculated to reveal a small difference between isostructural metal and semiconductor. The weak electron lobes distributed around the inter B6 octahedral bond were observed in the difference density. We found the electron lobes are the conductive p electrons in LaB6 from the comparison with the theoretical charge density. We have successfully visualized very small amount of conductive p electrons from X-ray charge density. Electron density distribution is now one of the most information-rich observable owing to the great improvement of experimental situation such as synchrotron X-ray source.