Hydrodynamic thermal transport in BNC<inf>2</inf>

The thermal transport properties of the recently synthesized high-pressure phase of BNC2 are investigated using the iterative solution of the Boltzmann transport equation with inputs from density functional theory calculations. The thermal conductivity of BNC2 is found to be extremely sensitive to pressure and the thermal conductivity at room temperature under 20 GPa pressure is over 1400 W/m-K which is more than 40 % higher than the corresponding value at 0 GPa. Similar to diamond, the origin of this extremely high thermal conductivity is rooted at large phonon group velocities, which gives rise to fluid-like hydrodynamic thermal transport in BNC2. However, unlike diamond, the large isotope disorder of boron atoms in BNC2 results in a large phonon-isotope scattering which renders hydrodynamic flow prevalent only in isotopically pure samples at length scales of up to 65 μm at 100 K.