Ionic high-pressure form of elemental boron

Abstract

Nature 457, 863-867 (2009) This Letter presents the results of high-pressure experiments and ab initio evolutionary crystal structure predictions, and found a new boron phase that we named c-B 28 . This phase is comprised of icosahedral B 12 clusters and B 2 pairs in a NaCl-type arrangement, stable between 19 and 89 GPa, and exhibits evidence for charge transfer (for which our best estimate is d < 0.48) between the constituent clusters to give (B 2 ) d1 (B 12 ) d2 . We have recently found that the same highpressure boron phase may have given rise to the Bragg reflections reported by Wentorf in 1965 (ref. 1), although the chemical composition was not analysed and the data (subsequently deleted from the Powder Diffraction File database) seems to not have been used to propose a structure model. We also note that although we used the terms 'partially ionic' and 'ionic' to emphasize the polar nature of the high-pressure boron phase and the influence this polarity has on several physical properties of the elemental phase, the chemical bonding in c-B 28 is predominantly covalent.We acknowledge N. Dubrovinskaia, L. Dubrovinsky, E. Yu Zarechnaya, Y. Filinchuk, D. Chernyshov, V. Dmitriev, A. S. Mikhaylushkin, I. A. Abrikosov & S. I. Simak for drawing these issues to our attention.

Electron-Deficient and Polycenter Bonds in the High-Pressureγ−B28Phase of Boron

“…They confirm a charge transfer between boron atoms, but numerical values are different from those obtained from the theoretical density [5]. Combined with the positive r 2 RCP and small RCP of B4-B4-B5 (Table I), they suggest the following mechanism of charge transfer which primarily involves the atoms B4 and B5 and which is in agreement with a simple electron count.…”

“…The existence as true polymorphs of boron has been established for rhombohedral boron [2], rhombohedral boron [3,4], and the recently discovered orthorhombic high-pressure modification boron [5][6][7]. All three modifications can be obtained at ambient conditions by quenching.…”

“…The crystal structure of orthorhombic -B 28 can be described as a distorted cubic closest packing of B 12 clusters with B 2 pseudopair units (dumbbells) placed at the octahedral sites ( Fig. 1) [5,6]. Chemical bonding in these solids has been rationalized in terms of polycenter bonds on the B 12 closo cluster and two-electron-two-center (2e2c) and twoelectron-three-center (2e3c) bonds between the clusters [1].…”

“…Explanation of this phenomenon requires a detailed analysis of chemical bonding between boron atoms [8,9]. Based on calculated electronic band structures, a partial ionic nature of bonding has been proposed for boron [10] and-more recentlyfor the high-pressure polymorph -B 28 , suggesting a charge separation between the icosahedral cluster and the dumbbell [5]. On the other hand, there is now a general agreement that bonding in -B 28 is predominantly covalent [5,7,[11][12][13].…”

“…A multipole refinement [14] has been carried out with the computer program XD [18], resulting in an excellent fit to the observed Bragg reflections with R F ¼ 0:0120 [19]. The starting point was the set of atomic coordinates of the five crystallographically independent atoms as taken from the literature [5][6][7]. The purity of the sample has been established previously [7] and also follows from the excellent fit to the diffraction data.…”