High-Pressure Softening of the Out-of-Plane A_2u(Transverse-Optic) Mode of Hexagonal Boron Nitride Induced by Dynamical Buckling

Abstract

We investigate the highly anisotropic behavior of the in-plane and out-of-plane infraredactive phonons of h-BN by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high quality single crystals show strict fulfillment of selection rules and an unusually long E 1u (TO) phonon lifetime. Accurate values of the dielectric constants at ambient pressure ε ⊥ 0 = 6.96, ε ⊥ ∞ = 4.95, ε ∥ 0 = 3.37, and ε ∥ ∞ = 2.84 have been determined from fits to the reflectivity spectra. The outof-plane A 2u phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30 • allow us to observe both the A 2uTO and LO modes. Pressure coefficients and Grüneisen parameters for all infrared-active modes are determined and compared with ab-initio calculations. While Grüneisen parameters are generally small in this layered crystal, the A 2u (TO) displays an exceptionally large and negative Grüneisen parameter that results in a widening of the type I hyperbolic region with increasing pressure. The softening of the A 2u (TO) mode is induced by a dynamical buckling of the flat honeycomb layers.

Pressure dependence of the interlayer and intralayer E2g Raman-active modes of hexagonal BN up to the wurtzite phase transition

“…Overall, there is good agreement between the experimental and GGA + wdW theoretical values for the pressure coefficients and Grüneisen parameters of both E 2g modes. The experimental values for the pressure coefficient and the Grüneisen parameter of the E high 2g mode are on the same order of those of the in-plane E 1u IR-active modes [24].…”

“…Then, the elastic contribution to scattering is enhanced and, as a consequence, the background signal increases significantly. Previous IR studies [24] indicated a somewhat higher transition pressure, around 13 GPa. The variation in the transition pressure obtained from these studies could be related to the different experimental techniques used and to the presence of non-hydrostatic tensions in the KBr PTM employed for the IR measurements.…”

“…For a precise calibration of the shifts in high-pressure data on bulk h-BN, fundamental studies of the high-pressure behavior of the Raman spectra of h-BN on high-quality crystals are required. Recently, the high-pressure behavior of the IR-active modes was studied by means of IR reflectance and absorption experiments [24], but Raman-scattering studies on high-quality h-BN are still lacking. High-pressure Raman-scattering experiments were reported on multiwalled h-BN nanotubes and the results were compared with Raman spectra of the intralayer E 2g mode obtained from low-quality pyrolytic h-BN bulk samples [25].…”

Boron nitride for excitonics, nano photonics, and quantum technologies

“…Since that, actors operating in different area of the science of devices for information technology have been rapidly coming into the play, and at the time being, hBN is world-widely used for different kinds of research. The bulk crystals have been of value for measuring the refractive indices [20], the stiffness coefficients [21], refining the Infra-Red signature of phonon energies [22] in polarization conditions [23] and they have triggered a lot of photonic applications in the near infrared [24]. These applications include: hyper-lensing effects useful in nanomedecine [25].…”

“…As a consequence there are very different hyperbolic polaritons in these two Reststrahlen bands. Advantage can be taken of: -The huge value of the dielectric constant to squeeze the wavelength of the infrared light for instance in the ∼12-∼6 µm range -The hyperbolic dispersion relation in the space of wavenumber to generate specific photonic effects [24][25][26][27][28][80][81][82][83] Using bulk crystals, and re-handling, the measurement on large single crystals we have recently measured the polarized spectral dependences of the dielectric constant that can be modelled using Lorentzian functions having significantly different values of the oscillator strengths and broadening parameters [22], giving in particular more huge negative values of the dielectric constant for wavelengths in the ∼6 µm range, as can be seen in Figure 6a, b. To the best of our knowledge, these huge negative values of epsilon that can only be generated in other semiconducting materials after expensive and tricky processing are not naturally observed for other bulk semiconductors of technological interest, in these wavelengths.…”