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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.

Ammonia borane, NH(3)BH(3), has attracted significant interest as a promising candidate material for hydrogen storage. The effect of pressure on the bonding in NH(3)BH(3) was investigated using Raman spectroscopy to over 20 GPa in a diamond anvil cell, and two new transitions were observed at approximately 5 and 12 GPa. Vibrational frequencies for the modes of the NH(3) proton donor group exhibited negative pressure dependence, which is consistent with the behavior of conventional hydrogen bonds, while the vibrational frequencies of the BH(3) proton acceptor group showed positive pressure dependence. The observed behavior of these stretching modes supports the presence of dihydrogen bonding at high pressure. In addition, the BH(3) and NH(3) bending modes showed an increase in spectral complexity with increasing pressure together with a discontinuity in d nu/d P which suggests rotational disorder in this molecule. These results may provide guidance for understanding and developing improved hydrogen storage materials.

Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg 2 SiO 4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373-1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress (σ) and specimen strain rates (ε . ) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and σ conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones.These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the "puzzling" seismic-anisotropy anomalies commonly observed in subduction zones.