High-pressure synthesis and thermal conductivity of semimetallic θ-tantalum nitride

The lattice thermal conductivity (κ${}_{ph}$) of metals and semimetals is limited by phonon-phonon scattering at high temperatures and by electron-phonon scattering at low temperatures or in some systems with weak phonon-phonon scattering. Following the demonstration of a phonon band engineering approach to achieve an unusually high κ${}_{ph}$ in semiconducting cubic-boron arsenide (c-BAs), recent theories have predicted ultrahigh κ${}_{ph}$ of the semimetal tantalum nitride in the θ-phase (θ-TaN) with hexagonal tungsten carbide (WC) structure due to the combination of a small electron density of states near the Fermi level and a large phonon band gap, which suppress electron-phonon and three-phonon scattering, respectively. Here, measurements on the thermal and electrical transport properties of polycrystalline θ-TaN converted from the ε phase via high-pressure synthesis are reported. The measured thermal conductivity of the θ-TaN samples shows weak temperature dependence above 200 K and reaches up to 90 Wm${}^{-1}$ K${}^{-1}$, one order of magnitude higher than values reported for polycrystalline ε-TaN and δ-TaN thin films. These results agree with theoretical calculations that account for phonon scattering by 100 nm-level grains and suggest κ${}_{ph}$ increase above the 249 Wm${}^{-1}$ K${}^{-1}$ value predicted for single-crystal WC when the grain size of θ-TaN is increased above 400 nm. (© 2023 Wiley‐VCH GmbH)