Novel Potassium Polynitrides at High Pressures

TL;DR

This study predicts and characterizes novel potassium polynitride structures stable at high pressures, revealing unique nitrogen arrangements and a metallic phase with potential for high energy density applications.

Contribution

It introduces new potassium polynitride compounds, including a metallic K₂N₁₆ with a fused nitrogen ring network, discovered through first-principles calculations.

Findings

Discovery of stable N₄, N₅, N₆ rings at high pressures

Identification of a metallic K₂N₁₆ with a nitrogen network

Pressure-dependent IR spectra for experimental guidance

Abstract

Polynitrogen compounds have attracted great interest due to their potential applications as high energy density materials. Most recently, a rich variety of alkali polynitrogens (R_{x}N_{y}; R=Li, Na, and Cs) have been predicted to be stable at high pressures and one of them, CsN_{5} has been recently synthesized. In this work, various potassium polynitrides are investigated using first-principles crystal structure search methods. Several novel molecular crystals consisting of N_{4} chains, N_{5} rings, and N_{6} rings stable at high pressures are discovered. In addition, an unusual nitrogen-rich metallic crystal with stoichiometry K_{2}N_{16} consisting of a planar two-dimensional extended network of nitrogen atoms arranged in fused eighteen atom rings is found to be stable above 70 GPa. An appreciable electron transfer from K to N atoms is responsible for the appearance of unexpected chemical bonding in these crystals. The thermodynamic stability and high pressure phase diagram is constructed. The electronic and vibrational properties of the layered polynitrogen K_{2}N_{16} compound are investigated, and the pressure-dependent IR-spectrum is obtained to assist in experimental discovery of this new high-nitrogen content material.