Superhyperfine structure in the EPR spectrum of ${\mathrm{Yb}}^{3+}$ in cubic sites of Ca${\mathrm{F}}_2$ has been investigated with EPR at both 9 Gc/sec and 35 Gc/sec and with ENDOR at 9 Gc/sec. The EPR measurements showed that the superhyperfine splitting is anisotropic and that the number of observable lines (17 at 9 Gc/sec along $〈100〉$) and the splitting of the lines is field-dependent. ENDOR measurements permitted complete interpretation of the EPR results. The superhyperfine structure arises from coupling to the nearest-neighbor fluorine nuclei. The Hamiltonian $\mathrm{S}·\mathrm{T}·{\mathrm{I}}^F$ is used to describe the ${\mathrm{Yb}}^{3+}$-${\mathrm{F}}^{-}$ interaction where T has axial symmetry. The principal values are|${\mathrm{T}}_{\mathrm{II}}$|=36.8±0.2 Mc/sec and|${\mathrm{T}}_{\mathrm{\perp }}$|=15.9±0.1 Mc/sec where ${\mathrm{T}}_{\mathrm{II}}$ and ${\mathrm{T}}_{\mathrm{\perp }}$ have opposite signs. The fluorine nuclear Zeeman term and the superhyperfine interaction are of the same order, resulting in a breakdown of the usual selection rules. The number, intensity, and splitting of the EPR superhyperfine lines have been calculated with these interaction parameters and they agree well with experiment. Small shifts in the fluorine ENDOR for ${}_{}{}^{171}{}_{}{}^{}\mathrm{Yb}(I=\frac{1}{2})$ and ${}_{}{}^{173}{}_{}{}^{}\mathrm{Yb}(I=\frac{5}{2})$ relative to the ENDOR for the even isotopes were observed which are due to the strong hyperfine coupling in the odd isotopes. Interpretation of ${\mathrm{T}}_{\mathrm{II}}$ and ${\mathrm{T}}_{\mathrm{\perp }}$ in terms of an isotropic and an anisotropic interaction is given, and results are compared with those of other workers.

• Received 30 August 1965

DOI:https://doi.org/10.1103/PhysRev.141.259