Electron Paramagnetic Resonance of Co-Doped PbWO4

The EPR of Co-doped PbW${\mathrm{O}}_4$ was observed at 9 GHz at temperatures below 16°K. Four sets of lines were seen, each characterized by an effective electronic spin $S$ of ½ and a nuclear spin $I$ of $\frac{7}{2}$. The spectral parameters of one set in an appropriate coordinate system were as follows: $g_i:4.50, 6.20, \mathrm{and} 2.07$; and $A_i\left(\mathrm{in}\mathrm{GHz}\right):0.605, 0.970, \mathrm{and} 0.248$ ($i=x, y, z$). The Eulerian angles of this coordinate system with respect to the crystal axis system ($\mathrm{caa}$) were: $\alpha =43.8\mathrm{}°$, $\beta =28.0\mathrm{}°$, and $\gamma =-65.3\mathrm{}°$. The remaining three cordinate sets are obtained by increasing $\alpha$ in increments of 90°. Above 16°K, the spectra gradually became broad and weak, and finally at 20°K disappeared, clearly as a result of the onset of fast spin-lattice relaxation. Below 4.2°K, the relaxation times became so long that difficulty was encountered in recording the true line shape. We believe that Co enters this lattice in a 2+ state and probably substitutes at a W site. The ground state in this site of $S_4$ symmetry then would be ${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$. From relaxation data, we estimate that the separation of the two sets of Kramers doublets is several tens of wave numbers. The EPR of Co-doped PbMo${\mathrm{O}}_4$ and PbW${\mathrm{O}}_4$ are essentially the same.