  Superconducting layered transition metal dichalcogenides (TMDs) stand out
among other superconductors due to the tunable nature of the superconducting
transition, coexistence with other collective electronic excitations (charge
density waves) and strong intrinsic spin-orbit coupling. Molybdenum disulphide
(MoS2) is the most studied representative of this family of materials,
especially since the recent demonstration of the possibility to tune its
critical temperature, Tc, by electric-field doping. However, just one of its
polymorphs, band-insulator 2H-MoS2, has so far been explored for its potential
to host superconductivity. We have investigated the possibility to induce
superconductivity in metallic polytypes, 1T- and 1T'-MoS2, by potassium (K)
intercalation. We demonstrate that at doping levels significantly higher than
that required to induce superconductivity in 2H-MoS2, both 1T and 1T' phases
become superconducting, with Tc = 2.8 and 4.6K, respectively. Unusually, K
intercalation in this case is responsible both for the structural and
superconducting phase transitions. By adding new members to the family of
superconducting TMDs our findings open the way to further manipulate and
enhance the electronic properties of these technologically important materials.
