Electron-nuclear double resonance ofCr3+53in guanidinium aluminum sulfate hexahydrate

Electron-nuclear-double-resonance (ENDOR) studies are reported for the isotope ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}_{}^{3+}{}_{}{}^{}$ enriched to 96% in guanidinium aluminum sulfate hexahydrate (GAlSH). The measurements were done at 4.2 °K and $X$-band microwave frequencies (∼9.4 GHz). The study allowed accurate values of the spin-Hamiltonian hyperfine parameters to be determined and ENDOR patterns to be classified. The sign of the zero-field splitting parameter $D$ was found to be negative, where previously only a positive relative value had been quoted. A linear relationship was found when plotting values of $Q^{\prime }$ vs $A-B$, utilizing data obtained in GAlSH and previous results of RbGa and CsGa alums. The best value of the quadrupole moment of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ was determined to be $Q=-0.028\mathrm{}\pm 0.005$ b.