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

ABSTRACTγ-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.