Discovery of γ-B28, a Novel Boron Allotrope with Partially Ionic Bonding

Abstract

ABSTRACTγ-B28is a recently discovered high-pressure phase of boron, with the structure consisting f icosahedral B12clusters and B2pairs in a NaCl-type arrangement: (B2)δ+(B12)δ-, and displaying a significant charge transfer δ~0.48. Boron is the only light element, for which the phase diagram has become clear only a few years ago, with the discovery of γ-B28, and this phase diagram is discussed here among other recent findings. γ-B28was first experimentally obtained as a pure boron allotrope in early 2004 by J.H. Chen and V.L. Solozhenko (although a similar diffraction pattern was published in a 1965 by R.H. Wentorf, in a paper that until recently was believed to be wrong) and its unique structure was discovered by A.R. Oganov in 2006 with the use of theab initioevolutionary algorithm USPEX (Oganov & Glass, 2006) and later confirmed by other studies. This allotrope, thermodynamically stable at high pressures, is shown to be also quenchable and dynamically stable upon decompression to 1 atm, and we show its phonon dispersion curves. Present discussion includes also the relative stability of other boron allotropes as a function of pressure. We also discuss more recent publications on the putative isosymmetric phase transition in γ-B28and the nature of chemical bonding in it. We demonstrate that a qualitative difference in the evolution of the band gap of γ-B28and the related α-B12structure, which is due to the partial ionicity of γ-B28.