北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

Stable armchairlike hexazine N6 ring in Tungsten Hexanitride – Drs. Nilesh Salke and Jin Liu

FEBRUARY 9, 2021


Tungsten hexanitride with armchairlike hexazine N6 ring has been synthesized by a group of scientists led by Dr. Jin Liu and his former postdoc Nilesh Salke at HPSTAR (Center for High Pressure Science & Technology Advanced Research). WN6 is a promising high-energy-density and superhard material. Their findings are published in the recent issue of Physical Review Letters (doi: https://doi.org/10.1103/PhysRevLett.126.065702).

Diatomic nitrogen is the most abundant molecule in Earth’s atmosphere accounting for almost 78% volume. The strong triple bond in nitrogen makes it very stable and unreactive at near ambient conditions. However, in the intense-pressure and high-temperature conditions, nitrogen will behave entirely differently, it can form double- or even single-bonded structure or react with other elements to form novel nitrides. Single-bonded polymeric nitrogen or nitrides possessing single-bonded nitrogen are of great scientific interest as a high-energy-density material. And transition metal nitrides are the very promising candidates that might contain the planar nitrogen hexazine (N6) ring which are predicted to be impossible to stabilize experimentally due to the lone pair repulsion.

The team created WN6 in a laser-heated diamond anvil cell by elemental reaction between tungsten and nitrogen above pressure of about 1.3 Mbar and temperature of ~3500 K. In-situ synchrotron x-ray diffraction (XRD) allowed them to identify the tungsten hexanitride phase, crystallizing with novel armchair-like N6 rings, and the high-pressure Raman spectroscopy measurement confirmed the presence of N-N single bonds in N6 rings. Further theoretical calculations also support their experimental observations.

"The armchair-like hexazine nitrogen sublattice in the WN6 is remarkable and comparable to that in the polymeric nitrogen phases, making it a promising high-energy-density material candidate,”remarked Dr. Nilesh Salke, now a postdoctoral researcher at the University of Illinois at Chicago.

 

Additionally, WN6 shows a Vickers hardness of up to∼57 GPa, the highest hardness among all transition metal nitrides along with good toughness. They credited the ultra-stiffness of WN6 to balance between the attractive interaction of N6 rings with W atoms and the repulsive interaction of N6 rings with each other based on theoretical calculations.

 

“To our knowledge, this is the first experimental report on the single-bonded transition metal nitride,” said Dr. Jin Liu, “We believe that this work will stimulate further experimental efforts to synthesize other nitrides with novel structural, chemical, and physical properties.”

“Our experimental demonstration of stabilizing armchairlike hexazine N6 ring in WN6 paves the way for future efforts to stabilize planar hexazine ring,”added Dr. Liu.


理论计算预测表明过渡金属六元氮化物中含有科学家一直寻找的平面型单键六元环氮,是一种潜在的高能量密度材料,且具有高熔点,高化学惰性,高压缩性,高硬度等优越的物理性能。然而这种被理论所预测的六元氮化物一直未能在实验上所证实。近日北京高压科学研究中心的刘锦研究员课题组与南京大学孙建教授,美国德克萨斯大学奥斯汀分校林俊孚教授合作的国际研究团队在高温高压下首次合成了过渡金属六元氮化物,WN6。他们发现WN6中的氮是以共价单键形式连接,形成了扶手椅状的氮六元环,是一种潜在的高能量密度材料。相关结果以“Tungsten Hexanitride with Single-bonded Armchairlike Hexazine Structure at High Pressure”为题发表于近日的《物理评论快报》。