ORDER AND DISORDER IN BORON PHASES

The complicated rhombohedral unit cell of crystalline beta-boron conta ining 105 atoms is a good approximant structure for a quasicrystal. Th us one may wonder if between the amorphous and the crystalline phases of boron, a non-metallic quasi-crystalline phase could exist. In order to examine this possibility and to characterize order and disorder in boron phases, we have performed high resolution electron microscopy ( HREM) and electron energy loss spectroscopy (EELS) measurements. In th e crystalline phase, the creation of large number of microtwins and st acking faults is the way that nature chooses to disorder boron. The pr esence of these microtwins seems to be the major obstacle towards the formation of a quasicrystal. A comparison between energy loss near edg e structures and electronic density of states for different boron comp ounds has allowed us to identify the signature of the icosahedra in EE LS measurements. In the amorphous phase, a special processing of HREM images proposed by Fan and Cowley has been applied in order to estimat e the extent of local order in amorphous boron. Both techniques (HREM and EELS) have confirmed the presence of icosahedral units in the amor phous phase. This raises the question of the existence of a quasi-crys talline phase.