高压下新型铁硼超导（超硬）材料的合成及其性质研究

Since the discovery of superconductivity in F-doped FeAs compounds in 2008, the exploration of new-type iron-based superconductors are of special interest. However, the discovered iron-based superconductors mainly exists in the typical parent (similar) structure of the FeAs or FeSe compounds at present. Recent Prof. Dubrovinskaia' experiments report the synthesis of an iron tetraboride (FeB4) and verify its superconductivity, revealing that the iron-boride compounds can be classified as the family of iron-based superconductors. In addition, the FeB4 compound was found to be superhard, reflecting that the iron-boride compounds also belong to the superhard material systems. This report arouses us tremendous interest especially in whether more iron-boride superconducting or superhard compounds would be synthesized under high pressure, due to high pressure as an efficient tool leading to form new compounds and phase with unusual properties for exotic materials. In this prospect, using the newly developed particle swarm optimization algorithm for crystal structure prediction combined with the first-principles calculations, the project intends to extensively explore the crystal structures of FexBy (x=1,2,3…; y=1,2,3…) in the 0-300 GPa ranges of pressure, build the high-pressure phase diagram, obtain the stable pressure and temperature range, explore the potential iron-based superconducting or superhard compounds. We expect that our calculations can provide important theoretical support for experimental works on synthesizing potential iron-boride superconducting or superhard compounds under high pressure.

自2008年日本研究组报道了在氟掺杂的铁砷化物中发现超导性质后，寻找新型铁基超导材料成为铁基超导领域的研究热点，但目前铁基超导材料主要停留在铁砷化物和铁硒化物等典型母体及类似结构中。最近Dubrovinskaia教授实验证实四硼化铁(FeB4)为一种常压新型铁基超导体，揭示出铁硼化合物为铁基超导材料的新结构体系。同时，该实验发现FeB4还具有超硬性质，表明铁硼化合物也属于超硬材料体系。高压能诱导新化合物和新结构相形成，高压Fe-B体系是否能合成更多新型的超导、超硬化合物成为关注的焦点。本项目拟利用基于粒子群优化算法的结构预测方法并结合基于密度泛函理论的第一性原理，系统探索FexBy(x=1,2,3…; y=1,2,3…)在0-300万大气压内的晶体结构，构筑高压相图，模拟实验合成条件，探索蕴含的新型铁硼超导、超硬材料。为实验上获得潜在新型铁硼超导、超硬等功能材料提供重要的理论支撑。

基于粒子群优化算法的结构预测方法和第一性原理研究，我们系统地研究了FexBy(x=1,2,3…; y=1,2,3…)在0-300 GPa的高压相图，确定了Fe-B体系中六种铁硼稳定化合物（Fe3B,Fe2B,FeB,Fe2B3,FeB2,FeB4）及其稳定结构.在这些稳定化合物中,研究发现其Fe原子配位数、Fe-B多面体堆积密度、B-B共价键数和材料结构硬度等随化合物中B的含量增加而增加。尤其，FeB2和FeB4表现出优越的力学性质，具有超硬潜力。深入研究FeB4化合物的高压相及其性质，揭示其结构相在0-1TPa区间从超导体半导体导体进行演化，研究表明这些性质与高压诱导形成的特殊硼结构单元及其电子-声子行为有关。本项目指出在Fe-B体系中FeB2、FeB4高压相都具有超硬性质，结构相Pnnm-FeB4具有超导性质，为实验探索Fe-B体系中超硬或超导材料提供了详实的指导，进一步指导我们开展了Fe-N，Fe-S等体系的高压研究。