Polytypism and Superconductivity in the NbS$_2$ System

TL;DR

This study systematically investigates the synthesis, phase identification, and superconducting properties of NbS$_2$ polytypes, emphasizing the importance of phase purity and accurate characterization in understanding their superconductivity.

Contribution

It provides a comprehensive synthesis map for NbS$_2$ polytypes and highlights the critical role of bulk measurements in identifying superconductivity.

Findings

2H-NbS$_2$ is a bulk superconductor

3R-NbS$_2$ is not superconducting above 1.75 K

Phase purity is essential for accurate property assessment

Abstract

We report on the phase formation and the superconducting properties in the NbS$_2$ system. Specifically, we have performed a series of standardized solid-state syntheses in this system, which allow us to establish a comprehensive synthesis map for the formation of the two polytypes 2H-NbS$_2$ and 3R-NbS$_2$, respectively. We show that the identification of two polytypes by means of X-ray diffraction is not always unambiguous. Our physical property measurements on a phase-pure sample of 3R-NbS$_2$, on a phase-pure sample of 2H-NbS$_2$, and a mixed phase sample confirm earlier reports that 2H-NbS$_2$ is a bulk superconductor and that 3R-NbS$_2$ is not a superconductor above $T =$ 1.75 K. Our results clearly show that specific heat measurements, as true bulk measurements, are crucial for the identification of superconducting materials in this and related systems. Our results indicate that for the investigation of van-der-Waals materials great care has to be taken on choosing the synthesis conditions for obtaining phase pure samples.