Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new ‘element’ turned out to be a compound containing less than 60–70% of boron, and it was not until 1909 that 99% pure boron was obtained1. And although we now know of at least 16 polymorphs2, the stable phase of boron is not yet experimentally established even at ambient conditions3. Boron’s complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions4,5 that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.
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A.R.O. acknowledges the Swiss National Science Foundation (grant 200021-111847/1) and the ETH Research Equipment Programme for support of this work. J.C. and T.Y. were supported by the NSF (grant EAR0711321) and the DOE (contract DE-FG02-07ER46461), Yanzhang Ma was funded by the DOE (agreement DE-FC03-03NA00144) and the NSF (grant DMR-0619215), and O.O.K. and V.L.S. were supported by the Agence Nationale de la Recherche (grant ANR-05-BLAN-0141). The use of the NSLS at Brookhaven National Laboratory was supported by the US Department of Energy under contract DE-AC02-98CH10886, and high pressure beamlines at the NSLS were supported by COMPRES under NSF cooperative agreement EAR 06-49658. Calculations were performed at CSCS (Manno), ETH Zurich, and the Joint Supercomputer Centre of the Russian Academy of Sciences.
Author Contributions A.R.O. did most of the calculations and wrote most of the paper, C.G. did most of the analysis of chemical bonding and wrote a significant part of the discussion, Yanming Ma contributed to calculations, C.W.G. wrote the first version of the USPEX code, J.C., V.L.S. and Yanzhang Ma synthesized the new phase, J.C. performed infrared absorption measurements with Z.L. and T.Y., V.L.S. did the Raman measurements and elemental analysis, V.L.S. and Yanzhang Ma did synchrotron X-ray diffraction measurements, and J.C. and O.O.K. performed the Le Bail refinement of the X-ray diffraction patterns.
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Oganov, A., Chen, J., Gatti, C. et al. Ionic high-pressure form of elemental boron. Nature 457, 863–867 (2009) doi:10.1038/nature07736
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