Paramagnetic Resonance of Divalent Holmium in the Cubic Hosts: Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Sr${\mathrm{Cl}}_2$

The paramagnetic-resonance spectra of the ground state of ${\mathrm{Ho}}^{2+}$ in the four lattices Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, Ba${\mathrm{F}}_2$, and Sr${\mathrm{Cl}}_2$ have been measured at helium temperatures at frequencies of 9 and 27 Gc/sec. The spectrum is isotropic and was fitted to a spin Hamiltonian which was modified to include a pseudonuclear $g$ factor which arises because of matrix elements with the nearest quartet state. The pseudonuclear $g$ factor permitted a determination of the energy separation to the quartet state. The $g$ value identified the ground state as ${\Gamma }_6$ for the fluorite lattices and ${\Gamma }_7$ for Sr${\mathrm{Cl}}_2$. It was found that the ratio of the crystal-field constants, $\frac{B_4}{B_6}$, decreases as the lattice spacing increases, which is contrary to what is expected from the point-charge model. The experimental $g$ values differ from the theoretical values by about 1.5%, and the discrepancy is attributed to orbit-lattice interaction and/or covalency. The magnetic hyperfine constant for the ${\mathrm{Ho}}^{2+}$ ion was found to be reasonably close to that reported for the Ho atom. However, the hyperfine constant for the Sr${\mathrm{Cl}}_2$ lattice differs from that in the fluorite lattices by about 1.5% and is not satisfactorily accounted for.