High Temperature Solid State Synthesis And Magnetic Properties Of BaFe₂Se₃ And BaFe₂Se₂O Single Crystals

Superconductivity is a kind of macroscopic quantum phenomenon.Because of its broad application prospect,it has been an important subject in the field of condensed matter physics since its discovery in 1911.The discovery of high-temperature superconductivity in copper-oxygen compounds in 1986 set off a boom in the study of high-temperature superconductors,but so far the understanding of its superconducting mechanism has not yet reached a consensus.The discovery of iron-based superconductors in 2008 was another significant advance after copper oxide high-temperature superconductivity,which provided a new platform for the study of superconductivity.For the research of superconducting mechanism of high-temperature superconductors,the most important is to study the superconducting phase diagram.The superconducting phase diagram includes three parts: antiferromagnetic state,pseudo-energy gap and superconducting state.If the antiferromagnetic states are doped by electrons or holes,it is possible to enter the superconducting region.For the 123 type iron-selenium compounds?AFe₂Se₃?and mixed anionic iron-selenium compounds,they mainly fall in the antiferromagnetic region.Therefore,the study of their structure and magnetic properties is helpful to understand the mechanism of high temperature superconductivity.In this paper,we study the 123 kinds of barium-iron-selenium compounds and barium intercalated iron-selenium compounds,the main contents include the following two aspects:?1?BaFe₂Se₃ single crystal was prepared by traditional high-temperature solid-state synthesis method.The structure and magnetic properties of BaFe₂Se₃ single crystal were characterized.The results of structural analysis of single crystal diffractometer showed that the crystal belonged to orthorhombic system,Pnma space group;X-ray Photoelectron Spectroscopy?XPS?measurement confirmed that iron ion of BaFe₂Se₃ single crystal existed in the form of Fe2+ and Fe3+.At the same time,we investigated the magnetic hysteresis loop and the temperature dependence of magnetization.The results indicated that the BaFe₂Se₃ single crystal had antiferromagnetic properties,and the Neel temperature was about 140 K.By summarizing the relationship between the Neel temperature and the average electron spin of the iron sites,we observed that the results of our samples were almost consistent with it.In addition,the magnetic measurements showed that there was a spin glass transition at the temperature of around 30 K.?2?The BaFe₂Se₂O single crystal with the Se site substituted by O was successfully synthesized by high temperature solid state method,and its crystal structure and magnetic properties were characterized.The results showed that the crystal belonged to orthorhombic system and Pmmn space group.The results of Energy Dispersive Spectrometer?EDS?measurement showed that the molar ratio of Ba,Fe and Se atoms in BaFe₂Se₂ O single crystal was close to 1: 2: 2,which was consistent with the target product.The study of its magnetic properties showed that the compound had antiferromagnetic properties at about 120 K and 20 K.In addition,there was some tilted ferromagnetism at low temperature.In general,we synthesized the single crystals of 123-type barium-iron-selenium compounds and barium intercalated oxyselenide with high-temperature solid-state method.By studying its structure and magnetic properties,we may provide new ideas and methods for exploring the new the iron-based superconductors.