A recent experimental study reported the successful synthesis of an orthorhombic FeB 4 with a high hardness of 62(5) GPa [Gou et al., Phys. Rev. Lett. 111,157002 (2013)], which has reignited extensive interests on whether transition-metal borides (TRBs) compounds will become superhard materials. However, it is contradicted with some theoretical studies suggesting transition-metal boron compounds are unlikely to become superhard materials. Here, we examined structural and electronic properties of FeB 4 using density functional theory. The electronic calculations show the good metallicity and covalent Fe-B bonding. Meanwhile, we extensively investigated stress-strain relations of FeB 4 under various tensile and shear loading directions. The calculated weakest tensile and shear stresses are 40 GPa and 25 GPa, respectively.Further simulations (e.g. electron localized function and bond length along the weakest loading direction) on FeB 4 show the weak Fe-B bonding is responsible for this low hardness. Moreover, these results are consistent with the value of Vickers hardness (11.7-32.3 GPa) by employing different empirical hardness models and below the superhardness threshold of 40 GPa. Our current results suggest FeB 4 is a hard material and unlikely to become superhard (> 40 GPa).2

Using a recently developed 'Crystal structure AnaLYsis by Particle Swarm Optimization' (CALYPSO) algorithm on a structural search, we predicted a novel sp(3) carbon allotrope possessing an orthorhombic lattice with the space group Cmmm (oC32). The calculated elastic constants and the simulated hardness revealed that oC32 simultaneously possesses ultra-incompressible and superhard properties with a high bulk modulus of 457 GPa and a high Vickers hardness of 96.2 GPa. This oC32 phase is dynamically stable and energetically more preferable than the experientially observed cold-compressed carbon, thus oC32 is expected to be experimentally synthesizable under extreme conditions. These results further expand the list of meta-stable carbon allotropes and superhard materials under atmospheric and extreme conditions.

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Polymeric nitrogen at high pressures has been proven to be a promising candidate for the design and discovery of high-energy-density materials as well as hard materials. Here, we report predicted high-pressure structures and physical properties of gallium nitrides using an effective unbiased structure searching method combined with first-principles calculations. Several unconventional stoichiometries of GaN 2 , GaN 5 , and GaN 8 compounds were predicted to be stable at a moderate pressure. The results reveal that GaN 5 with a N content of 83.3%, which is thermodynamically stable at 62.3 GPa by squeezing solid GaN and N 2 gas, is likely to be recovered as a metastable multifunctional material at ambient pressure. Given a computed energy density of ∼3.85 kJ/g, a Vicker's hardness of ∼30 GPa, and an indirect band gap of ∼2.1 eV, GaN 5 is a potentially functional material with high-energy-density, hard, and photovoltaic properties. The present results open a new avenue to synthesize multifunctional polynitrogen compounds.

The structure of I4̄-CsSi, a potential photovoltatic material and precursor of a superconducting silicon allotrope.