Spin-Hamiltonian Parameters as a Function of Pressure-Induced and Cation-Induced Changes in Nearest-Neighbor Distance for Europium-151 in Alkaline Earth Fluorides

The electron-paramagnetic-resonance spectra of the systems Ca${\mathrm{F}}_2$: ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}_{}^{2+}{}_{}{}^{}$, Sr${\mathrm{F}}_2$: ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}_{}^{2+}{}_{}{}^{}$, Ba${\mathrm{F}}_2$: ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}_{}^{2+}{}_{}{}^{}$ have been studied at a pressure of 26 kbar. These spectra were interpreted in terms of the usual phenomenological spin Hamiltonian with parameters $g$ (spectroscopic splitting factor), $b_4$ and $b_6$ (crystalline-field parameters), and $A$ (hyperfine splitting factor). The variations of $b_4$ and $b_6$ with pressure were compared with their variations with cation, and a consistent interpretation of the differences was found in terms of local lattice distortions about the impurity europium ions. This model leads to the relationships $b_4\propto {\delta }^{-8.8\mathrm{}}$, $b_6\propto {\delta }^{-13.6\mathrm{}}$ between the crystalline-field parameters and the nearest-neighbor distance $\delta$.