摘要: 通过在氮中引入杂质离子，利用高压手段获得具有新奇结构的多氮化合物是目前被广泛应用的研究方法。钙氮材料在催化、光电方面有着广泛的应用。具有较低电离能的钙（Ca）元素很容易和氮原子形成离子键钙氮化物。高压为寻找新型钙氮化合物提供了全新的技术途径。因此，利用高压方法，通过改变配比的方式，寻找具有新奇特性的钙氮高压结构，是一项非常有意义的工作。本文利用基于密度泛函理论的结构搜索方法，在100GPa条件下，通过预测得到了一个稳定的Ca5N4相。该结构内部氮原子之间以N-N共价单键键合，氮原子和钙原子之间是离子键相互作用，且钙氮之间的电荷转移量为1.26e/N atom。能带结构计算表明P21/c-Ca5N4是一个直接带隙为1.447eV的半导体结构。最后，系统地给出了该结构的拉曼振动光谱，并指认了拉曼振动模式，为实验合成该结构提供了理论指导。
First-principles study of Ca5N4 at high pressure
- Received Date:
28 November 2019
Abstract: Recent studies have shown that introducing metal elements into nitrogen matrix can induce more stable poly-nitrogen structures compared with the pure nitrogen phase, due to the ionic interaction between metal elements and nitrogen matrix. Many types of poly-nitrogen structures have been reported by applying the alkaline earth metal elements (M = Be, Mg, Ca, Sr, Ba) as the coordinate elements. For example, the 1D infinite armchair poly-nitrogen chains (N∞) structures and N6 ring structures are obtained for the MN4 and MN3 chemical stoichiometries, respectively. Interestingly, the stability of theses MNx structures has been enhanced 2-3 times compared with the pure nitrogen. Therefore, exploring the novel and stable poly-nitrogen structure by introducing alkaline earth metal elements under high pressure is a great significant work. As an alkaline earth element, Ca is abundant in the earth. Its ionization energy (I1=590 KJ/mol) is far lower than that of Be (900KJ/mol) and Mg (738KJ/mol), which means Ca can form calcium nitrides easier. Peng et.al has proposed that Ca-N system can obtain poly-nitrogen structures under high pressure, such as CaN4 structure with armchair nitrogen chain, CaN5 and CaN3 consisting of pentazolate “N5” and benzene-like “N6” anions. These poly-nitrogen structures have potential application prospects in the field of high energy density materials. Here, we report the prediction of Ca-N system at 100GPa by using particle swarm optimization (PSO) algorithm technique for crystal structure prediction. A new thermal stable phase with P21/c-Ca5N4 space group is found at 100GPa, which enriched the phase of Ca-N system under high pressure. The dynamic stability and mechanical stability of new phase are confirmed by phono dispersion spectrum and elastic constants calculations. ELF analysis shows that the nitrogen atoms in P21/c-Ca5N4 are bonded by N-N single bond and electron transfer from Ca atom to N atom enables Ca5N4 as an ionic-bonding interaction structure. Band structure calculation shows that Ca5N4 is a semiconductor structure with a 1.447eV direct band gap. PDOS calculation shows the valence band near Fermi energy is mainly contributed by N_p electrons, while the conduction band is mainly contributed by Ca_d electrons, indicating that electrons are transferred from Ca atom to N atom. Bader calculation shows that each N atom obtains 1.26e from Ca atom in P21/c-Ca5N4. The Raman spectrum is calculated and detailed Raman vibration modes are identified, which provides theoretical guidance for experimental synthesis.