1. Laboratory of Crystallography, Department of Materials, ETH Zurich, Wolfgang-Pauli-Str. 10, CH-8093 Zurich, Switzerland
   
2. Geology Department, Moscow State University, 119992 Moscow, Russia
   
3. Center for the Study of Matter at Extreme Conditions and Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33199, USA
   
4. Mineral Physics Institute and Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA
   
5. CNR-ISTM Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milano, Italy
   
6. Department of Mechanical Engineering, Texas University of Technology, 7th Street & Boston Avenue, Lubbock, Texas 79409, USA
   
7. National Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
   
8. Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA
   
9. LPMTM-CNRS, Université Paris Nord, Villetaneuse, F-93430, France
   
10. Present address: Department of Geosciences and New York Center for Computational Science, Stony Brook University, Stony Brook, New York 11794-2100, USA.

Correspondence to: Artem R. Oganov^1,2,10 Correspondence and requests for materials should be addressed to A.R.O. (Email: artem.oganov@sunysb.edu).

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 predictions^{4, 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.