Ionic high-pressure form of elemental boron

Boron is an element of fascinating chemical complexity. This arises from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons that could in principle favour metallicity, yet they are sufficiently localized to give rise to an insulating state. This delicately balanced electronic structure is easily modified by pressure, temperature and impurities, making it difficult to establish boron's structure and properties. Oganov et al. have now explored the high-pressure behaviour of boron and uncovered a previously unknown ionic phase consisting of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. This paper has explored the high-pressure behaviour of boron and uncovered a new phase that consists of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. 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.