The complex dielectric constant for erbium-doped strontium fluoride has been measured at pressures up to 0.4 GPa over the temperature range 300-360 K. Consequently, the reorientation of a type-II dipole (substitutional rare-earth and next-nearest-neighbor interstitial-flourine charge compensator) has been studied. The activation volume for the motion is found to be 4.73 ± 0.1 ${\mathrm{cm}}^3$/mol. It is argued that this value should be similar to the migration volume for "free" interstitials. Excellent agreement is found between a value for the migration volume calculated from Flynn's dynamical-diffusion model and the experimental value for bound interstitials. Finally, the compressibility of the activation volume is found to be more than an order of magnitude greater than the compressibility of the host lattice, and the thermal-expansion coefficient for the activation volume is found to be negative.

• Received 25 June 1979

DOI:https://doi.org/10.1103/PhysRevB.21.794