Novel high pressure structures and superconductivity of niobium disulfide

We have investigated the pressure-induced phase transition and superconducting properties of niobium disulfide (NbS₂) based on the density functional theory. The structures of NbS₂ at pressures from 0 to 200 GPa were predicted using the multi-algorithm collaborative (MAC) structure prediction technique. The previously known 1T-, 2H-, and 3R-NbS ₂ were successfully reproduced. In addition, many metastable structures which are potential to be synthesized were also discovered. Based on the enthalpy calculations, we found that at 26 GPa NbS₂ transits from the double-hexagonal (2H) structure to the tetragonal I4/mmm structure with a 10.6% volume reduction. The calculated elastic constants and phonon dispersion curves of I4/mmm-NbS₂ confirm its mechanical and dynamical stability at high pressure. More interestingly, the coordination number of Nb in I4/mmm structure is eight which is larger than that in the traditional metal dichalcogenides, indicating a new type of bondings of Nb and S atoms. In the new Nb-S bondings, one Nb atom and neighboring eight S atoms form a [NbS ₈] hexahedron unit. Furthermore, I4/mmm-NbS₂ exhibits a higher superconducting critical temperature than 2H-NbS₂, as is resulted from the stronger electron-phonon coupling coefficients.