Novel high pressure structures and superconductivity of niobium disulfide

Abstract

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 doublehexagonal (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, I4/mmm-NbS2 exhibits a higher superconducting critical temperature than 2H-NbS2, as is resulted from the stronger electron-phonon coupling coefficients.

Raman spectroscopy of transition metal dichalcogenides

“…While NbS 2 in the 2H phase does not show a CDW phase transition [41] [42], NbS 2 in the 3R phase shows a CDW phase transition [43]. Moreover, NbS 2 and NbSe 2 both show superconductivity below a critical temperature and pressure [40] [44].…”

Emergence of high superconductivity in a layered TaS3 crystal

“…To better understand the superconducting mechanism in the predicted P2 1 /m phase, we calculate the electron-phonon coupling (EPC) parameter l and the logarithmic average phonon frequency o log for TaS 3 at selected pressures from 20 to 200 GPa, as depicted in Table 1. We notice that l of the P2 1 /m phase reaches a relatively high value of 1.54 at 40 GPa and 1.97 at 50 GPa, which is much larger than that of superconducting NbS 2 (1.47) 45 and even comparable with some high-T c hydrides, such as H 3 S (2.19), 46 YH 6 (2.93), 47 etc., indicating a strong EPC in the high-pressure P2 1 /m phase. Therefore, a high superconductivity can be achieved in combination with the relatively small o log of 134.40 K at 50 GPa.…”

Random anion distribution in MSxSe2−x (M = Mo, W) crystals and nanosheets

“…First, we counted the metal atoms coordinated to at least one Se atom and compared that number to those that are coordinated to S atoms only. Statistically, if the Se atoms are randomly distributed, then at a composition of MS 1.8 Se 0.2 the probability that any M atom is coordinated to six S atoms is (0.90) 6 ¼ 0.53.…”

“…2 There is a need, especially in electronic devices, for nanosheets that have both non-zero band gaps and high carrier mobility. The physical properties of TMDs cover the range of semiconducting (WS 2 6 and ferromagnetic (Mn intercalated NbS 2 ), 7 and in this way they are complementary to those of graphene. In addition to electronic devices, there is also interest in using TMDs as photoelectrode materials, 8,9 catalysts, 10-12 and as platforms for studying charge 13 and phonon transport.…”