Electric Field Effect in Paramagnetic Resonance forMn2+in PbMoO4

We have measured the linear electric-field-induced shifts and some of the spin-Hamiltonian parameters for ${\mathrm{Mn}}^{2+}$ in PbMo${\mathrm{O}}_4$. The results are compared with those obtained earlier for ${\mathrm{Mn}}^{2+}$ in the related scheelite lattices CaW${\mathrm{O}}_4$ and SrW${\mathrm{O}}_4$. The site is axial with $S_4$ point symmetry in all cases. The $D$ term is, however, of opposite sign in PbMo${\mathrm{O}}_4$. The electric-field-effect parameters are about 60% as large in PbMo${\mathrm{O}}_4$ as in SrW${\mathrm{O}}_4$. This conflicts with a trend towards bigger shifts for ions in more spacious lattice sites which has been observed for Mn in CaW${\mathrm{O}}_4$ and SrW${\mathrm{O}}_4$ and for a number of other systems. It is suggested that this reversal of $D$ and of the general trend for the electric shifts may be due to large-amplitude rapid vibratory motion associated with incipient instability of the ${\mathrm{Mn}}^{2+}$ ion in PbMo${\mathrm{O}}_4$.