Pressure dependence of the infrared active modes in h-BN: Softening of the out-of-plane A2u(TO) mode

9th International Conference on Low Dimensional Structures and Devices in Puerto Varas (Chile), 2-6 December 2019.This work has been published in The Journal of Physical Chemistry C 123, 17491 (2019)Hexagonal boron nitride (h-BN) is attracting a great deal of attention in the emerging field of two-dimensional atomic crystals and van der Waals heterostructures because its extreme optical anisotropy [1] results in optical hyperbolic behavior in the mid-infrared. h-BN is particularly well suited for phonon-polariton applications because of the low optical losses, the strong and broad phonon resonances in the technologically relevant mid-infrared spectral region, and the concurrence of Type I and Type II hyperbolic response in the same material [2]. Considering the central role of the infrared (IR)-active phonons in limiting the phonon-polariton propagation length and the availability today of high-quality h-BN single crystals, we have revisited the polar phonons and dielectric properties of h-BN and we study the anisotropic pressure dependence of the in-plane and out-of-plane IR-active modes up to 10 GPa [3]. Infrared reflectivity spectra at normal incidence on high quality single crystals show strict fulfillment of selection rules and an unusually long E1u(TO) phonon lifetime. Accurate values of the dielectric constants at ambient pressure ¿0¿=6.96, ¿¿¿=4.95, ¿0||=3.37, and ¿¿||= 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A2u phonons are revealed in measurements on an inclined facet. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab-initio calculations. While Grüneisen parameters are generally small, the A2u(TO) mode displays an exceptionally large and negative Grüneisen parameter [3]. This unusual behavior results from the softening of the A2u(TO) mode due to a dynamical buckling of the flat honeycomb layers induced by the atomic motions. Such buckling instabilities play a central role in structural phase transitions of lamellar crystals [4]. This work has been supported by the Spanish MINECO/FEDER grant No. MAT2015-71035-R.This work has been funded by MINECO/FEDER under Projects MAT2015-71035-R and MAT2016-75586-C4-1-P