Elastic and photoelastic anisotropy of solid HF at high pressure

We report the results of a Brillouin scattering study of the elastic and photoelastic properties of solid HF as a function of pressure up to about 120 kbar. We performed near-forward-scattering experiments in which the single-crystal samples lie in a plane bisecting the scattering angle. These measurements, made in a miniature Merrill-Bassett diamond-anvil cell, yield the velocities of longitudinal- and transverse-acoustic phonons propagating along the b and a axes of single crystals of orthorhombic HF. Relative magnitudes of the elastic constants $c_{11}$, $c_{22}$, $c_{44}$, and $c_{66}$ are given as a function of pressure. We have also measured the velocity of the quasilongitudinal phonon propagating in the b-a plane as a function of angle at 25.4 kbar. This measurement determines the relative magnitude of $c_{12}$. The values of $c_{11}$/ρ, $c_{22}$/ρ, $c_{12}$/ρ, $c_{44}$/ρ, and $c_{66}$/ρ at 25.4 kbar (in units of ${10}^{10}$ ${\mathrm{cm}}^2$/${\sec }^2$) are 15.1, 28.5, 12.0, 9.74, and 2.56, respectively. We estimate the pressure dependence of the ratios of the squares of certain photoelastic constants from the intensity of the Brillouin peaks. Our results show that the polarizability derivatives associated with the shearing motion of chains of HF (and, consequently, the hydrogen bonds) are much larger than the polarizability derivatives associated with the compressional motion of the chains (and the hydrogen bonds). Both the elastic and photoelastic properties are quite anisotropic at low pressures, becoming more isotropic at higher pressure.