Effect of Sodium Substitution on Structural and Magnetic Properties of KFe2-y Se2

We have synthesized the superconducting K1−x Na x Fe2−y Se2 (x = 0.07 and 0.25) crystal by using the flux technique. SEM images demonstrate that two samples consist of successive plate-like surfaces oriented into ab plane. EDS analysis yields the average compositions as (Na0.20K0.80)0.96Fe1.78Se2for x = 0.07 and (Na0.33K0.67)0.96Fe1.73Se2 for x = 0.25. XRD pattern gives the (00 l) peaks. The main peaks are indexed to the I4/m space group corresponding to reflected intensities from (002), (006), (008), and (0010) planes related to the peaks of the phase K2Fe4Se5. The lattice parameters are calculated as a = 8.701(4) and c = 14.185(2) A for x = 0.07, and a = 8.689(3) and c = 14.054(4) A for x = 0.25. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperatures, T c , obtained from magnetization measurements are estimated to be 31.0 and 28.4 K for x = 0.07 and 0.25, respectively. The critical current value, J c (0), deduced from the M−H loops are approximately 2.8 ×104 A/cm 2 for x = 0.07 and 2.87 ×102 A/cm 2 for x = 0.25 at 5 K. The volume pinning force, F p , of x = 0.07 sample is found larger than that of the x = 0.25 one.