Putting xenon and nitrogen under pressure: towards new layered and two-dimensional nitrogen allotropes with crown ether-like nanopores

Abstract

Noble gas (Ng) compounds can be utilized to obtain novel structures of polynitrogen that cannot be directly attained under ambient conditions. Here, we report the prediction of several unique nitrogen allotropes via stable xenon–nitrogen compounds under pressure by using evolutionary algorithm searches in conjunction with first-principles calculations. Two-dimensional (2D) 18-crown-6 N₈ layers and three-dimensional (3D) sodalite-like N₂₀ polynitrogen are identified within XeN₈, XeN₁₈, and XeN₂₀ at 100 GPa, respectively. Notably, the 2D 18-crown-6 N₈ layers persist to ambient pressure on decompression after the removal of xenon, forming not only a layered bulk P N₈ phase, but also a P N₈ monolayer, which are dynamically and thermally stable. However, the recovery of the sodalite-like N₂₀ allotrope from Fm XeN₂₀ is unlikely, due to the larger size of the Xe atom compared to that of the channel pores in the 3D-N₂₀ net. Nevertheless, the calculations show that He and Ne are suitable for forming a stable Ng@N₂₀ at 100 GPa, ideal high-pressure precursors to produce the Fm 3D-N₂₀ allotrope. Furthermore, the estimated energy density for the layered bulk P N₈ (7.3 kJ g⁻¹) and sodalite-like N₂₀ (10.3 kJ g⁻¹) makes them stand out as promising high-energy density materials. This work is an effective effort to design and stabilize novel nitrogen allotropes with noble gases.