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.

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The strengthening of polycrystalline metals based on grain refinement has previously been reported to be no longer effective below a critical grain size of approximately 10-15 nm (Refs. 1, 2). That report imposed a limit on grain size tuning for synthesizing stronger materials. Here, we report our study using a diamond-anvil cell coupled with radial X-ray diffraction to track in situ the yield stress and deformation texturing of pure nickel samples with various average grain sizes. Continuous strengthening isobserved from 200 nm to the minimum grain size of 3 nm. Strengthening as a function of grain size is enhanced in the lower grain size regime below 20 nm. We achieved an ultra-high strength of ~ 4.2 gigapascals in nickel, 10 times larger than the values for commercial nickel material. The maximum flow stress of 10.2 gigapascals is reached in 3 nm nickel in the pressure range of this study. Plasticity simulation and transmission electron microscopy (TEM) examination reveal that the high strength observed in 3 nm nickel is caused by the superposition of strengthening mechanisms: partial and full dislocation hardening plus grain boundary plasticity suppression. These results rejuvenate the search for ultra-strong metals via materials engineering.Understanding the strengthening of nanograined metals has been puzzling, as both mixed results of size softening and hardening have been reported [3][4][5][6] . The main challenges in resolving this debate are the difficulty in synthesizing high quality, ultrafine metal samples for traditional tension or hardness tests and making statistically reproducible measurements. Some researchers have pointed out that reported size softening may be related to materials preparation 7 . Porosity, amorphous regions and impurities may be introduced during sample preparation methods like inert gas condensation and electrodeposition, leading to softening in

Unlike conventional reservoirs, tight reservoirs have complex pore structures and severe boundary-layer effect. The pore throat of tight reservoirs is in nanoscale and the boundary layer cannot be ignored because the boundary layer has an important effect to the fluid flow and its influence increases with the reduction in the pore throat radius. These are the main reasons for the ultra-low permeability and low oil recovery for these reservoirs. However, previous studies have paid limited attention to the influences of the boundary-layer effect and the pore size distribution. In this paper, a new model was built to determine the effective permeability for the tight gas and oil reservoirs by taking into account the boundary-layer effect and the pore distribution. The results from this new model show good agreement with the experiment data, and the main factors that impact the effective permeability were analyzed in the study. It is found that the fluid type, means and standard deviation of pore radius, and displacement pressure gradient are the main factors influencing effective permeability. The relationship of air permeability and liquid permeability is also analyzed for tight reservoirs.