Electron Spin Resonance of Gadolinium-Hydride and Gadolinium-Deuteride Ion Pairs in Calcium Fluoride Crystals

Electron-spin-resonance measurements of gadolinium-hydride and gadolinium-deuteride ion pairs of tetragonal symmetry in crystals of calcium fluoride are reported. The parameters of the spin Hamiltonian for the ground state of the ${\mathrm{Gd}}^{3+}$ ion are determined. It is shown that the main parts of the ${}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^6{}_{}{}^{}P_{\frac{7}{2}}^{}{}_{}{}^{}$ state splittings can be correlated and are due to the second-degree term in the crystal-field potential. The second-degree term is calculated using an "ionic" model including dipole moments of ions, and it is found to be in a reasonable accord with the experimentally derived value. Contributions to the isotope shift between the zero-field splittings of ${\mathrm{Gd}}^{3+}$ paired with hydride and with deuteride ions are calculated. The total value of the isotope shift estimated by using the ionic model appears to be one-quarter the magnitude of the observed shift and of opposite sign.