Superconducting Phases in Potassium-Intercalated Iron Selenides

摘要

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A_xFe_(2-y)Se_2 (A = K, Cs, Rb, TI/Rb, TI/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K_xFe_2Se_2(NH_3)_y (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K_(0.3)Fe_2Se_2(NH_3)_(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) A and K_(0.6)Fe_2Se_2(NH_3)_(0.37) to the 30 K phase with c = 14.84(1) A. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH_3 has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K_(0.3)Fe_2Se_2 and K_(0.6)Fe_2Se_2 SC phases. K_(0.3)Fe_2Se_2(NH_3)_(0.47) and K_(0.6)Fe_2Se_2(NH_3)_(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T_c with doping is not dome-like.