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X Feng, K Bao, Q Tao, L Li, Z Shao, H Yu, C Xu, S Ma, M Lian, X Zhao, Y Ge, D Li, D Duan, P Zhu and T Cui
Abstract
Recent reports exposed an astonishing factor of high hardness that the connection between transition-metal ™ atoms could enhance hardness, which is in contrast to the usual understanding that TM-TM will weaken hardness as the source of metallicity. It is surprising that there are two opposite mechanical characteristics in the one TM-TM bond. To uncover the intrinsic reason, we studied two appropriate mononitrides, CrN and WN, with the same light-element (LE) content and valence electron concentration. The two high-quality compounds were synthesized by a new metathesis under high pressure, and the Vickers hardness is 13.0 GPa for CrN and 20.0 GPa for WN. Combined with theoretical calculations, we found that the strong correlation of d electrons in TM-TM could seriously affect hardness. Thus, we make the complementary suggestions of the previous hardness factors that the antibonding d-electron state in TM-TM near the Fermi level should be avoided and a strong d covalent coupling in TM-TM is very beneficial for high hardness. Our results are very important for the further design of high-hardness and multifunctional TM and LE compounds.
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