Order and Disorder in Boron Phases

Abstract

Abstract. 2014 The complicated rhombohedral unit cell of crystalline 03B2-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

Atomic Structure and Vibrational Properties of IcosahedralB4CBoron Carbide

“…They distort the crystal only slightly because they preserve the rhombohedral symmetry and the structure of the icosahedron. In B 4 C ceramics obtained by hot pressing, however, these defects can be as close as a few lattice parameters [18], and the low-frequency part of the Raman spectrum is quite different from those of Fig. 3.…”

“…For this reason the apparent bulk modulus of the icosahedra, which was found smaller than B 0 , might need to be verified in a quasi hydrostatic environment, with an accurate measurement of pressure. Moreover, twinning in boron allows the crystal to accommodate large strains and to prevent the formation of quasi-crystalline phases [18]. We are inclined to attribute the observed anti-molecular compression to pressure inhomogeneities induced by the intensive twinning in B 4 C ceramics.…”

“…At higher frequencies, structural disorder is likely to be responsible for most of the observed broadening. Twinning or stacking faults-recently identified as pairs of neighboring twin defects-are observed in single crystals [18]. They distort the crystal only slightly because they preserve the rhombohedral symmetry and the structure of the icosahedron.…”

Quasicrystal approximants with novel compositions and structures

“…Notably, it contains a fairly large concentration of Boron. Owing to the prevalence of icosahedra in elemental Boron, researchers have long suggested that icosahedral quasicrystals might occur in Boron-rich compounds [13,14,15,16,17,18], but none has yet been found.…”

Discovery of a New Boron-Rich Phase by Electron Crystallography

“…Several reviews exist to cover the results obtained up to 1983 (Pearson, 1972;Matkovich, 1977;Kuz'ma, 1983). Boron also exists in an amorphous state which has been studied recently by transmission electron microscopy (Favia et al, 1996).…”