Electric Field Effect in Paramagnetic Resonance forMn2+in SrWO4

The electric field shifts in the paramagnetic resonance (EPR) for ${\mathrm{Mn}}^{2+}$ in SrW${\mathrm{O}}_4$ have been measured to be 104 and 225 for $R_{\perp }$ and $R_{\parallel }$, respectively, in units of Hz per V/cm. Comparison is made with the corresponding results obtained earlier for ${\mathrm{Mn}}^{2+}$ in CaW${\mathrm{O}}_4$. The experimental evidence indicates that the magnitude of the shifts depends on the ease with which the ${\mathrm{Mn}}^{2+}$ ion is displaced by the applied electric field, i.e., on the looseness of the ion in its substitutional site. In an earlier study involving the ${\mathrm{Yb}}^3$ ion in the same host lattices, it was found that "ionic-motion" effects of this kind could be explained in terms of new even crystal field components seen by the displaced ion. In the present case, however, the observed magnitudes were too large to be attributed to the motion-induced crystal field components. The polarization of the ${\mathrm{Mn}}^{2+}$ ion induced by the applied field appears to be the primary mechanism giving rise to the electric field EPR shifts; the ease of ionic displacement of the impurity ion entering into the result insofar as it enhances the magnitude of the electric field is actually seen at the ${\mathrm{Mn}}^{2+}$ site. This result is especially surprising in view of the fact that in the previous ${\mathrm{Yb}}^{3+}$ study, we found that polarization effects of the larger and better-fitting Yb ion were, by and large, negligibly small. It is suggested here that the classification of electric field effects as ionic or electronic is liable to be misleading and that it is more appropriate to distinguish electric shifts which involve odd-even-state interactions from those which do not. In either case, motion of the ions will play an important role.