Two electron-spin-resonance absorption spectra, each due to trivalent gadolinium, have been observed in single crystals of yttria, ${\mathrm{Y}}_2$${\mathrm{O}}_3$. These spectra were observed at room temperature, 77 K, and 4.2 K with a spectrometer operating at 0.85-cm wavelength. One of these spectra exhibits trigonal symmetry with its principal direction being parallel to a crystal body diagonal. The other spectrum exhibits orthorhombic symmetry whose three mutually orthogonal directions are along a crystal cube edge and two crystal face diagonals. Both spectra exhibit resolved hyperfine structure. Fine-structure and hyperfine-structure parameters are given for both spectra. For the orthorhombic gadolinium ion, a slight displacement in the fine-structure spectrum of ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$ from the spectrum due to the even-numbered isotopes is observed. This displacement can be expressed as a relative increase of about six parts in ${10}^4$ in the value of $B_2^0$. No analogous shift is observed in the trigonal spectrum of ${}_{}{}^{157}{}_{}{}^{}\mathrm{Gd}$.

• Received 10 August 1981

DOI:https://doi.org/10.1103/PhysRevB.25.1505