The origin of the vanadium dioxide transition entropy

TL;DR

This study uses density functional theory and machine learning to analyze the thermodynamic origin of the VO₂ metal-insulator transition, revealing phonon entropy as the dominant factor and accurately predicting the transition entropy.

Contribution

It introduces a novel method combining harmonic phonon calculations, experimental data, and Gaussian Process Regression to quantify phonon and electronic entropy contributions at the transition.

Findings

Phonon entropy drives five times more of the transition than electronic entropy.

Predicted total transition entropy accounts for 95% of calorimetric measurements.

Method accurately predicts thermodynamic properties of VO₂ transition.

Abstract

The reversible metal-insulator transition in VO$_2$ at $T_\text{C} \approx 340$ K has been closely scrutinized yet its thermodynamic origin remains ambiguous. We discuss the origin of the transition entropy by calculating the electron and phonon contributions at $T_\text{C}$ using density functional theory. The vibration frequencies are obtained from harmonic phonon calculations, with the soft modes that are imaginary at zero temperature renormalized to real values at $T_\text{C}$ using experimental information from diffuse x-ray scattering at high-symmetry wavevectors. Gaussian Process Regression is used to infer the transformed frequencies for wavevectors across the whole Brillouin zone, and in turn compute the finite temperature phonon partition function to predict transition thermodynamics. Using this method, we predict the phase transition in VO$_2$ is driven five to one by phonon entropy over electronic entropy, and predict a total transition entropy that accounts for $95$ % of the calorimetric value.