On the basis of the evolutionary methodology for crystal structure prediction, we have clarified the longstanding debates on the ground state structures of technically important molybdenum borides: MoB 2 , Mo 2 B 5 , and MoB 4 . The earlier proposed rhombohedral structure for Mo 2 B 5 and WB 4 -type structure for MoB 4 are ruled out. Instead, two novel hexagonal P6 3 /mmc structures are proposed. Moreover, the yet synthesized MoB 3 was found to adopt the intriguing rhombohedral R3 j m structure and was suggested to be experimentally synthesizable by the calculation of convex hull. Density of states calculation revealed that the strong covalent bonding between Mo and B atoms is the driving force for the high bulk and shear modulus as well as small Poisson's ratio of the studied borides. The hardness calculations suggest that these borides are all hard materials, among which MoB 3 exhibits the largest Vickers hardness of 31.8 GPa, exceeding the hardness of R-SiO 2 (30.6 GPa) and -Si 3 N 4 (30.3 GPa).

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Additional electrons can drastically change the bonding trend of light elements. For example, N atoms in alkali metal azides form the linear N3(-) anions instead of N2 molecules with the introduction of additional electrons. The effect of the additional electrons on the polymerization of N under pressure is important and thus far unclear. Using first principles density functional methods and the particle swarm optimization structure search algorithm, we systematically study the evolution of LiN3 structures under pressures up to 600 GPa. A stable structure featuring polymerized N under pressures higher than 375 GPa is identified for the first time. It consists of zig-zag N polymer chains that are formed by N5(-) five-member rings sharing N-N pairs. Throughout the stable pressure range, the structure is insulating and consists of N atoms in sp(3) hybridizations. Comparing with the atomic and electronic structures of previous phases, our study completes the structural evolution of LiN3 under pressure and reveals the structural changes which are accompanied and driven by the change of atomic orbital hybridization, first from sp to sp(2) and then from sp(2) to sp(3).

Manganese sulfide (MnS) nanocrystals (NCs) with three different phases were synthesized by one-pot solvent thermal approach. The crystal structures and morphologies were investigated using powder X-ray diffraction, transmission electron microscopy, and high-resolution transmission electron microscopy. We found that the crystal structure and morphology of MnS NCs could be controlled by simply varying the reaction temperature. The detailed growth process of MnS nanobipods, including the zinc blende (ZB)core formation and wurtzite (WZ)-arms growth, provides direct experimental evidence for the polymorphism model. Furthermore, we have studied the stability of metastable ZBand WZ-MnS NCs under high pressure and found that ZB-nanoparticles and ZB/WZ-nanobipods are stable below their critical pressure, 5.3 and 2.9 GPa, respectively. When pressures exceed the critical point, all these metastable MnS NCs directly convert to the stable rock salt MnS.