Discovery of γ-B 28 , a Novel Boron Allotrope with Partially Ionic Bonding

ABSTRACT

γ-B₂₈ is a recently discovered high-pressure phase of boron, with the structure consisting f icosahedral B₁₂ clusters and B₂ pairs 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-B₂₈ was 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 the ab initio evolutionary 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-B₂₈ and the nature of chemical bonding in it. We demonstrate that a qualitative difference in the evolution of the band gap of γ-B₂₈ and the related α-B₁₂ structure, which is due to the partial ionicity of γ-B₂₈.