Ionic high-pressure form of elemental boron (original) (raw)

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 obtained 1 . And although we now know of at least 16 polymorphs 2 , the stable phase of boron is not yet experimentally established even at ambient conditions 3 . Boron's complexities arise from frustration: situated between metals and insulators in the Periodic 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 B 12 clusters and B 2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and All known structures of boron contain icosahedral B 12 clusters, with metallic-like three-centre bonds within the icosahedra and covalent two-and three-centre bonds between the icosahedra. Such bonding satisfies the octet rule and produces an insulating state, but impurity-doped boron phases are often metallic. The sensitivity of boron to impurities is evidenced by the existence of unique icosahedral boron-rich compounds such as YB 65.9 , NaB 15 , MgAlB 14 , AlC 4 B 40 , NiB 50 and PuB 100 (refs 2, 6). In fact, probably only three of the reported boron phases correspond to the pure element 2,7,8 : rhombohedral α-B 12 and β-B 106 (with 12 and 106 atoms in the unit cell, respectively) and tetragonal T-192 (with 190-192 atoms per unit cell) 8 . At ambient conditions, α-B 12 and β-B 106 have similar static energies 9,10 , but disordered β-B 106 becomes marginally more stable (in what could seem a violation of the third law of thermodynamics) when zero-point vibrational energy is taken into account 10 . At pressures above several gigapascals, the much denser α-B 12 phase should be more stable at all temperatures. At high pressures, opposing effects come into play: although pressure favours metallic states and might stabilize metallic-like icosahedral clusters 11 , the very low packing efficiency of atoms in icosahedral structures (34% for α-B 12 ) necessitates the destruction of the icosahedra and formation of denser phases (for example, the α-Ga-type phase 12 ). In experiments, the room-temperature compression of β-B 106 showed metastable amorphization 11 at 100 GPa and the onset of superconductivity 13 at 160 GPa. When using laser heating to overcome kinetic barriers, it was found that β-B 106 transforms into the T-192 phase above 10 GPa at 2,280 K (ref. 14).