Spin-Lattice Coefficients for ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$ and for ${\mathrm{Gd}}^{3+}$ in CaO

We have measured the spin-lattice coefficients of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Gd}}^{3+}$ in Ca${\mathrm{F}}_2$ and ${\mathrm{Gd}}^{3+}$ in CaO by studying the effect of uniaxial stress on the EPR spectra of these ions. The experimental data show that fourth-order terms in the spin-lattice Hamiltonian give significant contributions. The values of the second-order spin-lattice coefficients are given, and the fourth-order contribution is tentatively identified as due to the change of the cubic field splitting with stress. From the value of the fourth-order coefficient, we conclude that at least half of the temperature dependence of the cubic field parameter is due to the effect of the lattice expansion.