新型FeS基层状化合物的合成、结构与物性研究

Iron chalcogenides have become the main research object of iron-based superconductors in recent years. Recently, we reported for the first time that tetragonal FeS is a superconductor with critical temperature of 5 K, which opens up a new field in studying iron-based superconductivity. The superconducting layer of tetragonal FeS is anti-fluorite [Fe2S2] layer, and therefore, according to the design concept of layered compounds, more new superconductors are expected to be built based on [Fe2S2] layer. At present, many research groups in the world are trying to explore new FeS-based layered compounds. However, the main factors restricting the development of FeS-based layered compounds are that these materials are difficult to synthesize and that the number of such materials reported is limited. In view of the above problems, the objective of this project is to explore new FeS-based layered compounds by developing new hydrothermal/solvothermal routes, and systematically study the phase formation, crystal structures and physical properties of these compounds. By intercalating various spacer layers in between [Fe2S2] layers, we can achieve carrier doping and induce structural distortion and finally tune the band structure and physical properties of [Fe2S2] layers. Through the study of this subject, we can deeply understand the relationship between the structure and physical properties of the FeS-based layered compounds and provide more new experimental platforms for the study of the superconducting mechanism of iron chalcogenides and for the development of related disciplines.

铁硫族化合物是近年来铁基超导领域的主要研究对象。近期，我们首次在国际上报道四方相FeS具有临界温度为5 K的超导电性，为铁基超导研究开辟了新的方向。FeS的超导电层是反萤石结构的[Fe2S2]层，根据层状化合物设计理念，基于[Fe2S2]层有望构建出更多新超导体。目前，国内外很多研究组都在努力探索新型FeS基层状化合物。而制约FeS基层状化合物发展的主要因素是该类材料合成困难，已报道的材料种类有限。针对上述问题，本项目将以探索新型FeS基层状化合物为目标，通过发展新的水热/溶剂热工艺，系统研究FeS基层状化合物的成相规律、结构和物性。通过在[Fe2S2]层间插入不同的空间层，以注入载流子，并产生结构畸变，进而达到调控[Fe2S2]层的能带和物性的目的。通过本课题的研究，可以深入认识FeS基层状化合物的结构和物性之间的关系，为铁硫族化合物超导机制的研究和相关学科的发展提供更多新的实验载体。

铁硫族层状化合物可以实现液氮温区以上的超导转变温度并且具有极其丰富的插层化学。目前研究的瓶颈主要是合成困难，种类有限。因此通过发展新的合成路径探索新型铁硫族层状化合物具有重要科学意义同时极具挑战性。本项目在分析铁硫族超导体结构化学和成相规律的基础上，设计具有超导关键结构单元FeS层的新化合物，取得的主要结果如下：.1）成功制备出了与FeSe基超导体类似的FeS基化合物Ax(NH3)yFe2S2 (A = Li，Na，K，Ba)和Ax(en)yFe2S2 (A = Li，Na；en = 乙二胺)，表征了结构和电磁性质。由于FeS母体的水分除不干净，同时FeS层的Fe极易被排出成为游离态的Fe，因此很难控制化合物的成分和物性，即使有时候观察到了超导迹象也很难重复。.2）合成了与FeS密切相关的化合物NaLiMS2 (M = Mn, Fe, Co)，它们具有CaAl2Si2类型结构，表现出半导体性质和自旋玻璃行为。其中NaLiCoS2的带隙适合吸收太阳光，光电流测试及开关响应曲线表明该材料有望用于光电器件。.3）合成了与(Li1-xFexOH)FeSe超导体结构类似的新化合物(Na1-xOH)Fe1-yX (X = S, Se)和(Na1-xOH)Fe0.5Fe0.5X (X = S, Se)。 (Na1-xOH)Fe1-yX由于结构畸变属于正交晶系，它们表现出顺磁性以及半导体导电性。第一性原理计算表明，(NaOH)FeSe的态密度、能带结构以及费米面结构都与(Li1-xFexOH)FeSe超导体相似，因此通过消除Fe空位有可能在(Na1-xOH)Fe1-y Se中实现超导电性。(Na1-xOH)Fe0.5Fe0.5X 与(Li1-xFexOH)FeSe超导体同构，表现出半导体性质和自旋玻璃行为，并且(Na1-xOH)Fe0.5Fe0.5S具有较好的锂电性能。.4）第一次在FeX (X = S, Se)中实现了N2H4的插层，获得了新化合物(N2H4)Fe2S2。XRD、TG、ATIR等测试结果验证了N2H4的插层，TEM以及Raman分析则表明其中的FeS层保持了反氧化铅结构。(N2H4)Fe2S2具有金属导电性和铁磁性，其中超导电性的消失与N2H4插层诱导的铁磁性密切相关。