112型铁基化合物EuFeAs2的单晶生长与表征

铁基超导体中含有一类特殊的112型结构化合物，其层状结构中含有一层锯齿形的As链构型.本文报道了用CsCl助熔剂法生长新型铁基112型EuFeAs₂母体单晶的具体方法，以及对该单晶的结构和物性的详细表征.通过能量色散X射线能谱扫描对单晶样品进行的化学成分分析，以及单晶X射线衍射的结构解析，确定该单晶样品属于EuFeAs₂相，结构精修得到EuFeAs₂具有空间群为Imm2（No.44）的正交晶体结构，晶格常数分别是a=21.285（9）Å，b=3.9082（10）Å，c=3.9752（9）Å.通过低温电阻测量，发现在110 K附近和46 K附近存在两个异常电阻跳变.进一步分析表明，110 K附近存在两个邻近的相变，这两个相变与铁基母体材料中常见的结构相变和Fe²⁺的反铁磁相变相符合.结合磁化率测量分析，可知46 K附近的相变属于Eu²⁺的反铁磁相变.

Single crystal growth and characterization of the 112-type iron-pnictide EuFeAs2

The 112-type (Ca, RE)FeAs₂ (RE=rare earth) superconductors are very special among the iron-based superconductors for their particular crystal structures with arsenic chain configuration and attractive electronic phase diagram with the coexistence of superconductivity and antiferromagnetism upon carrier doping, while the chemical phases are absent for the low doping level or undoped parent compound. Here we report the single crystal growth method and physical characterizations for the newly discovered Eu 112 type parent compound EuFeAs₂. The single crystal of EuFeAs₂ is grown by high temperature solution method through using CsCl as the flux under the constant temperature of 800℃ with the molar ratio of the starting materials Eu:Fe:As:CsCl=1:1:4:18. The as-grown crystal is shinyplatelike piece with a typical size of 1 mm×1 mm×0.2 mm, and quite stable in air. The chemical composition of EuFeAs₂ crystal is confirmed by energy-dispersive X-ray spectroscopy. The single crystal X-ray diffraction analysis at room temperature indicates that EuFeAs₂ crystallizes into an orthorhombic crystal structure with the space group Imm2 (No. 44), and the refined lattice parameters are a=21.285(9) Å, b=3.9082(10) Å, c=3.9752(9) Å, which are different from those of the Ca 112 compound, but similar to those of unique zigzag As-As chain configuration presented in the layered crystal structure. Electrical resistivity measurements show three anomalies near 110 K, 98 K, and 46 K. The former two anomalies with relatively high temperature imply that the structural and antiferromagnetic transitions are related to Fe²⁺ sublattice, which is similar to other iron-based parent compounds. The low temperature anomaly at 46 K is attributed to the antiferromagnetic transition of Eu²⁺ sublattice, which is also confirmed by the corresponding transition observed in the direct current magnetic susceptibility measurement. The magnetic susceptibility of EuFeAs₂ exhibits obvious anisotropy blow 46 K when the magnetic field is parallel or perpendicular to the bc plane, while the exact orientation of the Eu²⁺ moment needs further studying. The discovery of EuFeAs₂ provides a new platform for further studying the unique crystal structure and electronic state phase diagrams in the 112-type iron-based superconducting family, and may shed new light on the correlations between superconductivity and magnetism.