Novel Rubidium Poly-Nitrogen Materials at High Pressure

TL;DR

This study predicts and characterizes novel high-pressure rubidium poly-nitrogen compounds, including RbN5, RbN2, and Rb4N6, revealing their stability, structure, and potential synthesis pathways through first-principles calculations.

Contribution

The paper introduces newly predicted rubidium poly-nitrogen compounds stable at high pressures, expanding the understanding of nitrogen-rich materials and their synthesis.

Findings

RbN5 becomes stable above 30 GPa with aromatic N5- units.

RbN2 contains infinite nitrogen chains stable above 60 GPa.

Rb4N6 features N6 hexazine rings stable at high pressure.

Abstract

First-principles crystal structure search is performed to predict novel rubidium poly-nitrogen materials at high pressure by varying the stoichiometry, i. e. relative quantities of the constituent rubidium and nitrogen atoms. Three compounds of high nitrogen content, RbN_{5}, RbN_{2}, and Rb_{4}N_{6}, are discovered. Rubidium pentazolate (RbN5) becomes thermodynamically stable at pressures above \unit[30]{GPa}. The charge transfer from Rb to N atoms enables aromaticity in cyclo-N_{^{_{5}}}^{-} while increasing the ionic bonding in the crystal. Rubidium pentazolate can be synthesized by compressing rubidium azide (RbN3) and nitrogen (N2) precursors above \unit[9.42]{GPa}, and its experimental discovery is aided by calculating the Raman spectrum and identifying the features attributed to N_{^{_{5}}}^{-} modes. The two other interesting compounds, RbN2 containing infinitely-long single-bonded nitrogen chains, and Rb_{4}N_{6} consisting of single-bonded N_{6} hexazine rings, become thermodynamically stable at pressures exceeding \unit[60]{GPa}. In addition to the compounds with high nitrogen content, Rb_{3}N_{3}, a new compound with 1:1 RbN stoichiometry containing bent N_{3} azides is found to exist at high pressures.