By performing P doping on the Si sites in the topological semimetal Mo 5 Si 3 , we discover strong-coupling superconductivity in Mo 5 Si 3−x P x (0.5 ≤ x ≤ 2.0). Mo 5 Si 3 crystallizes in the W 5 Si 3 -type structure with space group of I4/mcm (No. 140), and is not a superconductor itself. Upon P doping, the lattice parameter a decreases while c increases monotonously. Bulk superconductivity is revealed in Mo 5 Si 3−x P x (0.5 ≤ x ≤ 2.0) from resistivity, magnetization, and heat capacity measurements. T c in Mo 5 Si 1.5 P 1.5 reaches as high as 10.8 K, setting a new record among the W 5 Si 3 -type superconductors. The upper and lower critical fields for Mo 5 Si 1.5 P 1.5 are 14.56 T and 105 mT, respectively. Moreover, Mo 5 Si 1.5 P 1.5 is found to be a fully gapped superconductor with strong electron-phonon coupling. First-principles calculations suggest that the enhancement of electron-phonon coupling is possibly due to the shift of the Fermi level, which is induced by electron doping. The calculations also reveal the nontrivial band topology in Mo 5 Si 3 . The T c and upper critical field in Mo 5 Si 3−x P x are fairly high among pseudobinary compounds. Both of them are higher than those in NbTi, making future applications promising. Our results suggest that the W 5 Si 3 -type compounds are ideal platforms to search for new superconductors. By examinations of their band topologies, more candidates for topological superconductors can be expected in this structural family.

Arsenic doping in silicides has been much less studied compared with phosphorus. In this study, superconductivity is successfully induced by As doping in Mo 5 Si 3 . The superconducting transition temperature (Tc) reaches 7.7 K, which is higher than those in previously known W 5 Si 3 -type superconductors. Mo 5 Si 2 As is a type-II BCS superconductor with upper and lower critical fields of 6.65 T and 22.4 mT, respectively. In addition, As atoms are found to selectively take the 8h sites in Mo5Si2As. The emergence of superconductivity is possibly due to the shift of Fermi level as a consequence of As doping, as revealed by the specific heat measurements and first-principles calculations. Our work provides not only another example of As doping, but also a practical strategy to achieve superconductivity in silicides through Fermi level engineering.