Prediction of extremely high thermal conductivity in a layered oxide near instability

Generally, materials near instabilities have low thermal conductivity. However, we predict that semiconducting layered ${\mathrm{NiO}}₂$, which is experimentally marginally stable and is based on chemically unstable tetravalent Ni, has an extremely high 300 K thermal conductivity within its layers, approximately 4 times that of sapphire and even approaching the thermal conductivity of beryllia, the highest thermal conductivity oxide. The explanation for this result is in terms of the unusual bonding of the ${\mathrm{NiO}}₂$ layers and the resulting phonon dispersions and anharmonicities. Specifically, the longitudinal acoustic and one of the transverse acoustic branches are bunched, related to unusual bonding for a transition metal oxide, while the other transverse acoustic branch is very soft. This leads to a low scattering phase space and high in-plane thermal conductivity.