Formation of Xenon-Nitrogen Compounds at High Pressure - Dr. Ross Howie
OCTOBER 17, 2016
A group led by HPSTAR scientist Dr Ross Howie and including Dr Jack Binns (RTH Lab) and Dr Phillip Dalladay-Simpson (CEP), in collaboration with researchers from CSEC (Edinburgh, UK) used high pressures to explore the possibility of forcing two of the more unreactive elements of the periodic table, xenon and nitrogen, to react. Joint investigation by X-ray diffraction and Raman spectroscopy showed the formation of a novel van der Waals compound at pressures as low as 5 GPa. After transformation to a lower symmetry phase this material, Xe(N2)2, remains remarkably stable up to at least 180 GPa and temperatures of 2000 K. This study is published in the journal Scientific Reports (doi:10.1038/srep34896).
Xenon, one of the inert gases, has closed shell system, assuming to be non-reactive under normal conditions. But over the years, researchers have explored altering the reactivity of xenon under high pressure with materials such as ice and oxygen as well as metals such as iron, nickel, and magnesium. These studies have attempted to provide an explanation for the significant abundance of xenon detected in the Earth’s atmosphere. A recent theoretical study predicted the formation of a novel xenon nitride compound at pressures above 146 GPa, a particularly interesting prediction given the well known inertness of both elements.
Caption: (Left) Crystal structures of Xe(N2)2 in high- and low-pressure phases. Xenon atoms are grey, nitrogen molecules and atoms are blue. (Right) Vibrational Raman spectra of Xe(N2)2 up to 175 GPa illustrating the persistence of N-N bonding.
“Both nitrogen and xenon are considered unreactive and inert at ambient conditions. Nitrogen has one of the strongest known triple bonds and xenon atoms possess a closed-shell electronic structure, not the typical properties required for forming stable compounds.” said Dr Binns. “The strongest interactions between xenon and nitrogen atoms in this compound are van der Waals forces, typically thought to be very weak.” Ross explained.
However this study shows such compounds can be far more stable than expected. In the low-pressure phase Xe(N2)2 adopts a cubic Laves type structure typically observed in metallic alloys. Further pressurization leads to N2 molecules orienting in one direction and forming a new high-pressure tetragonal phase. Diffraction peaks for this phase were visible up to 103 GPa, the very highest pressures achieved during X-ray studies.
Raman spectroscopy provided clear evidence of weakening nitrogen-nitrogen bonding above 60 GPa, however this intramolecular bond persisted to 180 GPa and heating to 2000 K, ruling out the formation of the predicted XeN6. Optical transmission measurements on samples of xenon-rich and nitrogen-rich mixtures showed that ‘doping’ Xe with Xe(N2)2 significantly reduces the metalization pressure below that of the constituent elements.
氙气是典型的惰性气体，氮气具有非常稳定的共价键，常压条件下他们都难以和其它元素发生化学反应而形成化合物。然而近几年人们发现在高压下，氙气及氮气都会变得活泼，能够与贵金属等反应生成化合物。最近的一篇理论研究表明在压力高于146 GPa （146万个大气压）下，氙气能与氮气发生化学反应，形成N-Xe 化合物。 在此理论预言的启发下，Howie研究员带领的科研团队进行了高压实验的研究。在他们多次实验的努力下，在大约5万个大气压件下发现了理论所预言的Xe-N化合物-Xe(N2)2。此化合物具有典型的立方Laves相结构。在更高的压力14万个大气压下，立方结构的-Xe(N2)2会逐渐转变为四方结构。此四方结构相对比较稳定，能保持四方结构到达180万个大气压，1700摄氏度条件下。