Wang Shanmin, Antonio Daniel, Yu Xiaohui, Zhang Jianzhong, Cornelius Andrew L, He Duanwei, Zhao Yusheng
HiPSEC &Physics Department, University of Nevada, Las Vegas, Nevada 89154, USA.
Institute of Atomic &Molecular Physics, Sichuan University, Chengdu 610065, China.
Sci Rep. 2015 Sep 3;5:13733. doi: 10.1038/srep13733.
Transition-metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock-salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10-20 GPa. Here, we report high-pressure synthesis of hexagonal δ-MoN and cubic γ-MoN through an ion-exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 - 80 μm. Based on indentation testing on single crystals, hexagonal δ-MoN exhibits excellent hardness of 30 GPa, which is 30% higher than cubic γ-MoN (23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo-N network than that in cubic phase. The measured superconducting transition temperatures for δ-MoN and cubic γ-MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.
过渡金属（TM）氮化物是一类具有广泛性质和应用的化合物。硬质超导氮化物对于诸如涂层和高应力等工作条件下的电子应用（例如机电系统）特别有吸引力。然而，大多数已知的TM氮化物以岩盐结构结晶，这种结构不利于抵抗剪切应变，并且它们表现出相对较低的压痕硬度，通常在10 - 20 GPa范围内。在此，我们报告了通过在3.5 GPa下的离子交换反应高压合成六方δ-MoN和立方γ-MoN。最终产物为块状，微晶尺寸为50 - 80μm。基于对单晶的压痕测试，六方δ-MoN表现出约30 GPa的优异硬度，比立方γ-MoN（约23 GPa）高30%，并且是迄今为止已知金属氮化物中最硬的。六方相硬度的提高归因于比立方相更广泛的共价键合Mo-N网络。测得的δ-MoN和立方γ-MoN的超导转变温度分别为13.8 K和5.5 K，与先前的测量结果吻合良好。
