Raman-scattering study of high-pressure effects on the anisotropy of force constants of hexagonal boron nitride

Frequency shifts of two species of Raman-active modes in layered hexagonal boron nitride have been measured in the pressure range from 1 bar to 110 kbar at room temperature. From the observed shifts it was found that, while the increase of an intralayer force constant of the crystal was only six percent, the interlayer shear force constant increased by four times as much at 110 kbar compared with the value at 1 bar. The pressure effects on the mode Grüneisen constants of the two distinct modes and on the shear elastic constant $c_{44}$ of the crystal are discussed.