Elastic anisotropy and lattice anharmonicity in hexagonal boron nitride revealed by birefringence-resolved Brillouin light scattering

Hexagonal boron nitride (h-BN) is a foundational van der Waals crystal whose exceptional mechanical anisotropy is pivotal for next-generation electronics and quantum technologies. However, a comprehensive understanding of its full elastic tensor and its temperature evolution, crucial for predictive device design, has remained incomplete. Here, we employ birefringence-resolved Brillouin light scattering to determine the elastic tensor and its temperature evolution in a single c-axis-oriented crystal h-BN. Using a circular polarization configuration, we simultaneously resolve the ordinary and extraordinary beams, which enables unambiguous disentanglement of birefringence-induced acoustic mode splitting. This sample-efficient approach yields three independent elastic constants at room temperature: C33 = 24.5 ± 0.1 GPa, C44 = 13.2 ± 0.9 GPa, and C13 = −3.1 ± 5.7 GPa, revealing the extreme mechanical anisotropy of this layered material. From these constants, the lattice thermal conductivity is estimated to be 21 W m−1 K−1 along the c-axis and 347 W m−1 K−1 within the basal plane, quantitatively reflecting the giant anisotropy in heat transport. Furthermore, temperature-dependent measurements across 65–460 K reveal a linear thermal softening of the out-of-plane elastic constant C33, characterized by a relative coefficient dC33/C33dT ≈ −3 × 10–4 K−1, alongside a concurrent linear increase in phonon linewidth governed by three-phonon scattering. These results provide essential quantitative data on the thermo-elastic properties of h-BN, crucial for the predictive modeling and reliable design of h-BN-based devices under thermal stress.