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_xN_y; R = Li, Na, and Cs) have been predicted to be stable at high pressures and one of them, CsN₅ 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 N₄ chains, N₅ rings, and N₆ rings stable at high pressures are discovered. In addition, an unusual nitrogen-rich metallic crystal with stoichiometry K₂N₁₆ consisting of a planar two-dimensional extended network of nitrogen atoms arranged in fused 18 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 K₂N₁₆ compound are investigated, and the pressure-dependent IR spectrum is obtained to assist in experimental discovery of this new high-nitrogen content material.

中文翻译：

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新型高压聚氮化钾

多氮化合物由于其作为高能量密度材料的潜在应用而引起了极大的兴趣。最近，据预测，多种多样的碱金属多氮化合物（R _x N _y； R = Li，Na和Cs）在高压下稳定，其中之一是最近合成的CsN ₅。在这项工作中，使用第一性原理晶体结构搜索方法研究了各种聚氮化钾。发现了由在高压下稳定的N ₄链，N ₅环和N ₆环组成的几种新型分子晶体。此外，化学计量比为K ₂ N ₁₆的不寻常的富氮金属晶体发现由排列在稠合的18个原子环中的氮原子的平面二维扩展网络组成的结构在70 GPa以上是稳定的。从K原子到N原子的明显电子转移导致这些晶体中出现意外的化学键。绘制了热力学稳定性和高压相图。研究了层状多氮K ₂ N ₁₆化合物的电子和振动性质，并获得了压力依赖性的红外光谱，以帮助实验发现这种新的高氮含量材料。