High‐Pressure Synthesis and Thermal Conductivity of Semimetallic θ‐Tantalum Nitride

Abstract 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.