Order and disorder in boron phases

The complicated rhombohedral unit cell of crystalline beta-boron containing 105 atoms is a good approximant structure for a quasicrystal. Thus 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 the crystalline phase, the creation of large number of microtwins and stacking faults is the way that nature chooses to disorder boron. The presence of these microtwins seems to be the major obstacle towards the formation of a quasicrystal. A comparison between energy loss near edge structures and electronic density of states for different boron compounds has allowed us to identify the signature of the icosahedra in EELS measurements. In the amorphous phase, a special processing of HREM images proposed by Fan and Cowley has been applied in order to estimate the extent of local order in amorphous boron. Both techniques (HREM and EELS) have confirmed the presence of icosahedral units in the amorphous phase. This raises the question of the existence of a quasi-crystalline phase.