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

The lattice thermal conductivity (kappa(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 kappa(ph) in semiconducting cubic-boron arsenide (c-BAs), recent theories have predicted ultrahigh kappa(ph) of the semimetal tantalum nitride in the theta-phase (theta-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 theta-TaN converted from the epsilon phase via high-pressure synthesis are reported. The measured thermal conductivity of the theta-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 epsilon-TaN and delta-TaN thin films. These results agree with theoretical calculations that account for phonon scattering by 100 nm-level grains and suggest kappa(ph) increase above the 249 Wm(-1) K-1 value predicted for single-crystal WC when the grain size of theta-TaN is increased above 400 nm.