Hyperfine, Superhyperfine, and Quadrupole Interactions ofGd3+in YPO4

The electron-paramagnetic-resonance spectrum of ${\mathrm{Gd}}^{3+}$ in YP${\mathrm{O}}_4$ shows a superhyperfine structure from two nearest-neighbor phosphorus nuclei when observed with $\overrightarrow{\mathrm{H}}$ parallel to the crystal $c$ axis. The spectrum for $\overrightarrow{\mathrm{H}}$ perpendicular to the $c$ axis is unusual because of quadrupole interactions in ${\mathrm{Gd}}^{155}$ and ${\mathrm{Gd}}^{157}$ which are larger than the hyperfine interaction in these ions. The measured ${\mathrm{Gd}}^{3+}$-P interaction parameter $\left|T_{\parallel }\right|=1.\mathrm{} 02$ G fits well a dipole-dipole interaction model. The hyperfine parameters are $A^{155}=-4.\mathrm{}7$ G and $A^{157}=-6.1\mathrm{}$ G. The quadrupole-interaction parameters are $\left|Q^{155}\right|=18.\mathrm{} 1$ G and $\left|Q^{157}\right|=19.\mathrm{}3$ G. From these the ratio of the nuclear quadrupole moments of ${\mathrm{Gd}}^{155}$ and ${\mathrm{Gd}}^{157}$ is obtained as 0.94 ± 0.01. This result is discussed in the light of quadrupole moments previously measured by optical and nuclear methods.