Pressure-induced 1T to 3R structural phase transition in metallic VSe2: X-ray diffraction and first-principles theory

TL;DR

This study investigates the pressure-induced structural phase transition in VSe2, revealing an isostructural anomaly at 4 GPa and a first-order transition from 1T to 3R phase at 11 GPa, combining experimental XRD and first-principles calculations.

Contribution

It provides a detailed analysis of the 1T to 3R phase transition in VSe2 under pressure, highlighting the importance of enthalpic corrections in DFT to accurately predict transition pressures.

Findings

Anomalies at ~4 GPa indicating isostructural transition.

First-order 1T to 3R phase transition at ~11 GPa.

Significant increase in Se atom anharmonicity across transition.

Abstract

We study pressure-induced structural evolution of vanadium diselenide (VSe2), a 1T polymorphic member of the transition metal di-chalcogenide (TMD) family using synchrotron-based powder X-ray diffraction (PXRD) and first-principles density functional theory (DFT). Our XRD results reveal anomalies at P ~4 GPa in c/a ratio, V-Se bond length and Se-V-Se bond angle signalling an isostructural transition. This is followed by a first order structural transition from 1T (space group P-3m1) phase to a 3R (space group R-3m) phase at P ~11 GPa due to sliding of adjacent Se-V-Se layers. We present various scenarios to understand the experimental results within DFT and find that the 1T to 3R transition can be captured only after inclusion of enthalpic correction associated with errors in cell volume with underestimated transition pressure. The abrupt increase in the Debye-Waller factors of Se atoms by a factor of ~4 and hence the anharmonic effects across the structural transition pressure are hitherto not reported so far and hint a possible way to understand the mismatch between the experimental and theoretical transition pressure values.