Electron Spin Resonance ofGd3+andEu2+in Single Crystals of CaO

Single crystals of CaO doped with ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Gd}}^{3+}$ have been investigated by paramagnetic resonance at $X$ band. Measurements were made at 290, 77, and 4.2°K. The spectra of ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Gd}}^{3+}$ can be described by a cubic spin Hamiltonian. The constants for the spectrum of ${\mathrm{Gd}}^{3+}$ at 290°K are $g=1.9913\mathrm{}\pm 0.0005$, $b_4=(+11.6\mathrm{}\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $b_6=(-1.15\mathrm{}\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The over-all splitting of the ground state is (380±4)×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The constants for the spectrum of ${\mathrm{Eu}}^{2+}$ at 290°K are $g=1.9914\mathrm{}\pm 0.001$, $b_4=(+24.0\mathrm{}\pm 0.5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $b_6=(-1.6\mathrm{}\pm 0.5)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $\left|A^{151}\right|=(29.6\mathrm{}\pm 0.1)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\left|A^{153}\right|=(13.05\mathrm{}\pm 0.2)\times {10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The over-all splitting of the ground state is (781±20)×${10}^{-4\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The splitting parameters are found to vary only slightly between 290 and 4.2°K.