Na-doping effects on structural evolution and superconductivity in (K_1-x Na _x )₂Cr₃As₃ (x  =  0-1)

In this report, we studied the effects of isovalent Na-doping on the recently discovered quasi-one-dimensional Cr-based unconventional superconductor K₂Cr₃As₃. A series of polycrystalline samples with nominal component (K_1-x Na _x )₂Cr₃As₃ (x  =  0-1) were synthesized by the solid state reaction method. From crystal structure and chemical phase characterizations, we found two distinct chemical phases with the same hexagonal structure but distinguished by different site occupancy of Na⁺ ions at the two kinds of K-site in the K₂Cr₃As₃ lattice structure. When x  ⩽  0.4, the doped samples form a continuous sosoloid phase of (K_1-x Na _x )₂Cr₃As₃ with the Na⁺ ions randomly doping at the K-sites (denoted as α-phase); when x  ⩾  0.5, a novel individual phase of (K_0.25Na_0.75)₂Cr₃As₃ emerges, in which the Na⁺ ions selectively occupy all the '3k' sites and the K⁺ ions occupy the '1c' sites (denoted as β-phase). No chemical phase of Na₂Cr₃As₃ was detected. Superconductivity in these samples was studied by electrical transport and magnetic susceptibility measurements, and it evolves in a much sophisticated manner. In the α-phase, the superconducting T _c decreases quickly upon Na-doping. All these α-phase samples have surprisingly low superconducting volume fraction and relatively low T _c compared with the undoped K₂Cr₃As₃. However, the β-phase has a clearly enhanced T _c up to 7.6 K which locates between the values of K₂Cr₃As₃ and Na₂Cr₃As₃, and exhibits a full superconducting shielding signal.