Radial diffraction strength and elastic behavior ofCaF2in low- and high-pressure phases

Abstract

The radial-diffraction lattice behavior of ${\text{CaF}}_{2}$ was analyzed in its low-pressure (fluorite) and high-pressure phase up to 11.5 GPa using radial x-ray diffraction techniques in the diamond anvil cell. Between 3.5 and 7.1 GPa, fluorite develops a radial-diffraction strength of $\ensuremath{\sim}0.8\text{ }\text{GPa}$. The corresponding lattice anisotropy of the fluorite phase was measured to be equal to 0.73, in good agreement with previous Brillouin spectroscopy measurements. By 8.8 GPa, ${\text{CaF}}_{2}$ has undergone a phase transformation to its high-pressure (orthorhombic) phase, with a corresponding volume decrease of 10.4%. By 11.5 GPa, the volume drop between the low-pressure and high-pressure phase has increased to 11.5%. In addition, the high-pressure phase is found to withstand a significantly larger differential stress than the low-pressure fluorite phase, with a large degree of lattice anisotropy. In the maximum stress direction at 8.8 GPa, we observe a time-dependent evolution of the lattice parameters of ${\text{CaF}}_{2}$, indicating that the high-pressure structure is still undergoing deformation on time scales of hours after the phase boundary has been crossed.