Electric-Field-InducedgShifts for Loose Yb Ions in Three Scheelite Lattices

The electric-field $g$ shift of ${\mathrm{Yb}}^{3+}$ in the scheelite series CaW${\mathrm{O}}_4$, SrW${\mathrm{O}}_4$, and BaW${\mathrm{O}}_4$ has been measured. In SrW${\mathrm{O}}_4$, and especially in BaW${\mathrm{O}}_4$, the Yb ion is much smaller than the cation, and we find a substantial increase in the $g$ shifts, which indicates less restraint to ionic motion. The lattice restoring forces for loose ions are investigated through the use of the Born-Mayer theory of ionic bonding. We find that the predicted ionic $g$ shift is considerably larger than that observed. This may be due to substantial local distortion near a Yb impurity. Several other mechanisms for increasing the restoring forces on small ions are also considered.