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Polynitrogen compounds have attracted great interest due to their potentialapplications as high energy density materials. Most recently, a rich variety ofalkali polynitrogens (R_{x}N_{y}; R=Li, Na, and Cs) have been predicted to bestable at high pressures and one of them, CsN_{5} has been recentlysynthesized. In this work, various potassium polynitrides are investigatedusing first-principles crystal structure search methods. Several novelmolecular crystals consisting of N_{4} chains, N_{5} rings, and N_{6} ringsstable at high pressures are discovered. In addition, an unusual nitrogen-richmetallic crystal with stoichiometry K_{2}N_{16} consisting of a planartwo-dimensional extended network of nitrogen atoms arranged in fused eighteenatom rings is found to be stable above 70 GPa. An appreciable electron transferfrom K to N atoms is responsible for the appearance of unexpected chemicalbonding in these crystals. The thermodynamic stability and high pressure phasediagram is constructed. The electronic and vibrational properties of thelayered polynitrogen K_{2}N_{16} compound are investigated, and thepressure-dependent IR-spectrum is obtained to assist in experimental discoveryof this new high-nitrogen content material.

Novel Potassium Polynitrides at High Pressures