Electric field effect ofGd3+-metal-ion complexes in CaF2

Electric-field-effect experiments on some EPR lines of orthorhombic ${\mathrm{Gd}}^{3+}$ — $M^{+}$ complexes in Ca${\mathrm{F}}_2$ are reported. We have investigated the effect of lattice distortions induced by the excess negative charge of the $M^{+}$ ion on the electric field effect; also the effective charge of this impurity has been taken into account. From a theoretical treatment we conclude that, with the relaxations proposed in an earlier paper by Bijvank, den Hartog, and Andriessen, one is able to predict the behavior of the electric field effect as a function of the $M^{+}$ radius.