Influence of Sn doping on the phase formation and superconductivity of FeSe_0.93

Sn doped FeSe_0.93 bulk samples were prepared by solid-state reaction in Ar atmosphere. The result suggests that Sn does not actually enter the crystal lattice of β-FeSe, existing as an inclusion SnSe instead. Although the existence of impurities such as Fe₇Se₈, Fe₃O₄ and SnSe is harmful to the superconductivity of β-FeSe, the T_c of the Sn doped FeSe_0.93 samples is not suppressed compared to the undoped one, quite different from previous studies on the metal doped FeSe superconductors. According to the thermal analysis, Sn was firstly melt and then reacted with Se forming SnSe. The presence of SnSe can obviously accelerate the subsequent formation of iron selenide. XRD result indicates that Sn addition can also promote the growth of β-FeSe grains along (0 0 1), (1 0 1), (1 1 2) crystal planes and expedites the transformation from Fe₇Se₈ to β-FeSe, the superconducting phase. Morphology observation also displays enlarged β-FeSe grains and their better connection with each other in the Sn doped samples. All the factors mentioned above are contributed to the relative high T_c of Sn-doped FeSe_0.93 samples. The present work suggests that Sn might be a promising element to improve the superconductivity of FeSe _1-x if the amount of addition as well as the sintering process is further optimized.