Origin of the transition entropy in vanadium dioxide

Abstract

The reversible metal-insulator transition in VO2 at Tc≈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 Tc 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 Tc using experimental information from diffuse x-ray scattering at high-symmetry wave vectors. Gaussian process regression is used to infer the transformed frequencies for wave vectors 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 VO2 is driven 5 to 1 by phonon entropy over electronic entropy, and predict a total transition entropy that agrees (within 5%) with the calorimetric value.