Anomalous high thermal conductivity in heavy element compounds with van der Waals interaction

It is conventionally believed that lattice thermal conductivity (κ) decreases with increasing atomic mass (negative atomic-mass correlation), and the high κ can only occur in crystals composed of strongly bonded light elements. By solving the fundamental thermal conductivity equation using first-principles calculations, here we reveal the anomalous κ departing from the long-held concept, that is, a positive atomic-mass correlation and high κ with heavy elements and weakly bonded interaction. We demonstrate this anomalous phenomenon by performing calculations of the cross-plane κ of the layered compounds, i.e., the h-BX family with X = N, P, and As. We find that the anomalous increase in the cross-plane κ with X going from N to As results in the cross-plane/in-plane conductivity ratio, generally expected to be much smaller than 1 in layered compounds, reaching as large as 2.6 at low temperatures. We also find that the unusually high cross-plane κ (660 W m-1 K-1), which is comparable to the bulk silicon with strong covalent bonding interactions, can be generated by a weak van der Waals interaction. Our analysis shows that the anomalous κ arises from one-dimensional-like phonons propagating in the cross-plane direction, which is due to the extremely large phonon anisotropy induced by the combined effect of atomic-mass difference and structural anisotropy. This discovery paves an avenue to realize thermally conductive materials that have weakly bonded structures, which can be potentially applied in the design of high-performance nanoelectronic devices.