Stress dependence of theEu2+nuclear quadrupole splitting and transferred hyperfine interaction in fluorite crystals

We have made electron-nuclear-double-resonance (ENDOR) measurements of the uniaxial stress-induced nuclear quadrupole splitting for ${\mathrm{Eu}}^{2+}$ in Ba${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ca${\mathrm{F}}_2$, and Cd${\mathrm{F}}_2$. For stress in the [001] direction the results are in agreement with predictions based on point-charge calculations. In the case of stress along the [111] direction, agreement was achieved only if (i) an anticipated stress-induced dipole contribution to the quadrupole splitting was neglected, and (ii) allowance was made for the local compressibility differing from the bulk compressibility.