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-NbS2 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 [NbS8]hexahedron unit. Furthermore, 14/mmm-NbS2 exhibits a higher superconducting critical temperature than 2H-NbS2, as is resulted from the stronger electron-phonon coupling coefficients. (C) 2014 Elsevier B.V. All rights reserved.