The crystal structure of a simple rhombohedral form of boron

A new modification of boron, resulting from the pyrolytic decomposition of BI z on a heated surface (800 C. to 1100 C.), has been found to have a simple rhombohedral structure, with a 0 = 5.057 /~ and c~ = 58.06 , space group R3m, Z ----12. The atomic parameters have been obtained from a single crystal using the precession method. The structure may be described as composed of units of nearly regular icosahedra in a slightly deformed cubic close packing. This requires the use of three-center or delta bonds, wherein two electrons are shared by three boron atoms at the vertices of an equilateral triangle.