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We report the discovery and detailed investigation of superconductivity in Mo4Ga20As. Mo4Ga20As crystallizes in a space group of I4/m (No. 87), with the lattice parameters a = 12.86352 Å and c = 5.30031 Å. The resistivity, magnetization, and specific heat data reveal Mo4Ga20As to be a type-II superconductor with T c = 5.6 K. The upper and lower critical fields are estimated to be 2.78 T and 22.0 mT, respectively. In addition, electron-phonon coupling in Mo4Ga20As is possibly stronger than the BCS weak-coupling limit. First-principles calculations suggest the Fermi level being dominated by the Mo-4d and Ga-4p orbitals.

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 (K1-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 (K1-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 (K0.25Na0.75)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.