Abstract
K$_{2}$Fe$_{4}$Se$_{5}$ is a newly discovered compound with superconductivity. It was found that the antiferromagnetism coexist with the superconductivity and is also related to the iron vacancy order. This gives a great opportunity to study the mystery of superconductivity. However, among the researches of iron vacancy order, there is no study using soft X-ray diffraction on K$_{2}$Fe$_{4}$Se$_{5}$ with high q resolution. In this thesis, we present the study of iron vacancy order of the K$_{2}$Fe$_{4}$Se$_{5}$ superconductor by using soft X-ray absorption and diffraction. Fe L$_{2,3}$-edge absorption data indicate that Fe atoms lose two electron and form Fe$^{2+}$ in K$_{2}$Fe$_{4}$Se$_{5}$. Furthermore, the diffraction results of momentum transfer (3/5, 1/5, 2) show that, as the temperature increases, the diffraction intensity starts to decrease at 545K and vanishes above 553K. In addition, the half width half maximum (HWHM) minimizes at 549K. In other words, the transition temperature of iron vacancy order is 549K. We also found a enhancement of iron vacancy order at 35K, a temperature close to the transition temperature(T$_{c}$) of superconductivity. That is, our results indicate competing orders in K$_{2}$Fe$_{4}$Se$_{5}$ superconductor.