Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan.
Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan.
Dalton Trans. 2018 Oct 7;47(37):12964-12971. doi: 10.1039/c8dt02387e. Epub 2018 Aug 29.
Iron-based superconductors can be categorized into two types of parent compounds by considering the nature of their temperature-induced phase transitions; namely, first order transitions for 122- and 11-type compounds and second-order transitions for 1111-type compounds. This work examines the structural and magnetic transitions (ST and MT) of CaFeAsH by specific heat, X-ray diffraction, neutron diffraction, and electrical resistivity measurements. Heat capacity measurements revealed a second-order phase transition that accompanies an apparent single peak at 96 K. However, a clear ST from the tetragonal to orthorhombic phase and an MT from the paramagnetic to the antiferromagnetic phase were detected. The structural (T) and Néel temperatures (T) were respectively determined to be 95(2) and 96 K by X-ray and neutron diffraction and resistivity measurements. This small temperature difference, T-T, was attributed to strong magnetic coupling in the inter-layer direction owing to CaFeAsH having the shortest lattice constant c among parent 1111-type iron arsenides. Considering that a first-order transition takes place in 11- and 122-type compounds with a short inter-layer distance, we conclude that the nature of the ST and MT in CaFeAsH is intermediate in character, between the second-order transition for 1111-type compounds and the first-order transition for other 11- and 122-type compounds.
铁基超导体可以根据其温度诱导相变的性质分为两类母体化合物；即 122-和 11-型化合物的一级相变和 1111-型化合物的二级相变。这项工作通过比热、X 射线衍射、中子衍射和电阻率测量研究了 CaFeAsH 的结构和磁转变（ST 和 MT）。热容测量显示出二级相变，伴随着在 96 K 处出现明显的单峰。然而，在四方相到正交相和从顺磁相到反铁磁相的 ST 和 MT 都被检测到。通过 X 射线和中子衍射以及电阻率测量，分别确定结构（T）和奈尔温度（T）为 95(2)和 96 K。由于 CaFeAsH 的晶格常数 c 在母体 1111-型铁砷化物中最短，因此这种小的温度差 T-T 归因于层间方向的强磁耦合。考虑到短层间距离的 11-和 122-型化合物中发生一级相变，我们得出结论，CaFeAsH 的 ST 和 MT 的性质介于 1111-型化合物的二级相变和其他 11-和 122-型化合物的一级相变之间，具有中间特征。
