Hexagonal boron nitride (h-BN) exhibits a wide indirect bandgap of 5.955 eV, as recently demonstrated by phonon-assisted emission and absorption measurements [1]. h-BN has attracted an enormous interest as an alternative dielectric for electronic devices based on van der Waals heterostructures resulting from stacking grapheme and transition metal dichalcogenide layers, because its surface is atomically flat and it is free of trapped charges. In addition, the thermal conductivity in the basal plane of the h-BN crystal is two orders of magnitude higher than that for the conventional SiO2 dielectric, and can be beneficial for the thermal management of these novel devices. Despite the current interest in h-BN, phonon dynamics in this material has not been thoroughly investigated. The analysis of the temperature dependence of Raman spectra can provide valuable information about the phonon decay processes in crystals. We have carried out a systematic study of both Raman active modes over a temperature range from 80 to 600 K. Whereas the low-energy mode (E2glow) corresponds to a gliding motion of the rigid hexagonal layers and is thus governed by weak van del Waals interactions, the high-energy mode (E2ghigh) probes the strong in-plane interatomic interactions. The high structural anisotropy of h-BN is reflected in its thermal expansion coefficient, which is negative in the basal plane and positive along the c direction. This has a strong influence on the E2glow and E2ghigh Raman frequencies. The frequencies and linewidths of the E2g modes are analyzed on the basis of Cowley¿s second-order perturbation theory. Density functional theory calculations of the phonon dispersion are carried out to identify the main phonon decay channels. On account of the lack of efficient decay channels, the E2glow mode is extremely narrow and exhibits weak anharmonic interactions (negligible broadening), its frequency downshift being mainly a consequence of thermal expansion. In contrast, the E2ghigh mode displays a substantial broadening that can be accounted for by a dominant 4-phonon decay process. Decay processes, however, are not sufficient to reverse the strong frequency upshift caused by the lattice thermal contraction and thus explain the observed E2ghigh downshift. Similarly to the case of the E2g mode of graphite [2], the contribution of the first-order 4-phonon scattering term is found to be dominant. This is related to the low-lying modes of the layered structure of h-BN and is confirmed by ab-initio estimations of the fourth derivative of the total energy with respect to the phonon coordinates. A good agreement is found by fitting the anharmonic model to the experimental data using the anharmonic coupling potentials as free parameters

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