Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS$_2$

TL;DR

This study investigates high-purity 1T-VS$_2$ samples, revealing no charge density wave (CDW) despite latent instabilities, and suggests electron localization and phase separation due to disorder or correlations, contrasting with previous Li-deintercalated samples.

Contribution

The paper provides new insights into the structural and electronic properties of 1T-VS$_2$, showing the absence of CDW in pure samples and proposing a phase separation mechanism.

Findings

No CDW observed down to 94 K in high-purity samples

Latent CDW instability indicated by phonon calculations

Evidence of electron localization and phase separation

Abstract

We report on a systematic study of the structural, magnetic and transport properties of high-purity 1T-VS$_2$ powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS$_2$. First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, \textit{ab initio} phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector ${\bf q}_{CDW} \approx$ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with \textit{ab initio} band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS$_2$ is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.