Structure and physical properties of iron-selenide K_xFe_2-ySe₂

We have synthesized the superconducting K_xFe_2-ySe₂ (x = 0.8) crystal by using the one step technique. SEM images demonstrate terrace-like structures on the surface and the sample contains two phases. XRD pattern gives the (00ℓ) peaks. The main peaks are indexed to the I4/m space group and the lattice parameter c is calculated to be 13.94076 Å. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperature, Tcmag, obtained from magnetization measurement is estimated to be 31 K. The critical current value, Jc(0), deduced from the M-H loops for sample is approximately 9 × 10⁴ A/cm² at 5 K. From magnetoresistivity measurement the T_c, Tconset and Tcoffset temperatures at the absence of the external magnetic field are found as 32.2, 33.1 K and 31.5 K, respectively. The calculated flux pinning or activation energy U is around 0.46 eV for 0 T.