Electron-Nuclear Double Resonance ofCr3+53in Gallium Alums. II

Electron-nuclear-double-resonance (ENDOR) studies are reported for the isotope ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}_{}^{3+}{}_{}{}^{}$ enriched to 96% in the gallium alums RbGa${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·12${\mathrm{H}}_2$O and CsGa${\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·12${\mathrm{H}}_2$O, including their deuterated forms. All measurements were made at 4.2°K and at $X$-band microwave frequencies (∼ 9.4 GHz). The study allowed accurate values of the various hyperfine parameters to be determined and ENDOR patterns to be classified. The hyperfine parameters $A$ and $B$ had the relationships $A>B$ in RbGa alum and $A<B$ in CsGa alum. A model for the behavior of the alums as a function of temperature is postulated, with the result that the quadrupole moment of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ is determined to be $eQ=-0.035\mathrm{}\pm 0.005 \mathrm{b}$. The magnetic moment of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}_{}^{3+}{}_{}{}^{}$ was found to be approximately $-0.464\mathrm{}{\mu }_N$ which is slightly lower than the accepted value. The effect of deuteration did not show significant differences from the hydrated alums. ENDOR transitions are observed relating three adjacent hyperfine levels of ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$ instead of the usual two. Possible mechanisms for this effect are qualitatively discussed.