Transition from electron-dominated to phonon-driven thermal transport in tungsten under extreme pressures

By utilizing a combination of first-principles calculations and machine-learning-interatomic-potential-based molecular dynamics simulations, we show that (contrary to the conventional understanding of electron-driven thermal transport in metals) the dominant carriers of heat in tungsten are phonons at high pressures. More specifically, we show that the contribution of phonons to the total thermal conductivity increases monotonically from $\ensuremath{\sim}30%$ at ambient pressure to $\ensuremath{\sim}70%$ at 100 GPa. This is attributed to considerable phonon hardening leading to enhanced phonon lifetimes and group velocities in pressurized tungsten. In contrast to the phonon-driven thermal conductivity, calculations of the electronic thermal conductivity based on density functional perturbation theory calculations of the full electron-phonon coupling matrix show that there is negligible change in the electron-driven thermal conductivity throughout the entire pressure range (up to 100 GPa) studied in this paper. Spectrally resolved electron-phonon coupling at elevated pressures reveals that while phonons are hardened, the peaks of the spectral function are unchanged, thus resulting in negligible variation of the mass enhancement parameter that describes the strength of the overall electron-phonon coupling with increasing pressures. In contrast, the characteristically reduced phonon-phonon scattering, a signature of the isotropic and highly degenerate phonon branches, leads to the phonon-dominated heat conduction in pressurized tungsten. Taken together, our results show that the pressure-driven changes in thermal transport can result in large deviations of the Lorenz number from the traditionally accepted Sommerfeld value in tungsten. Our work reveals an efficient way to separately manipulate the phononic thermal transport from the electronic heat conduction through the application of external pressure in tungsten, and as such can be highly beneficial for applications such as in integrated circuits where tungsten could replace the widely used copper interconnects.