Paramagnetic Resonance of ${\mathrm{Gd}}^{3+}$ in ${\mathrm{Al}}_2$${\mathrm{O}}_3$

The electron paramagnetic resonance spectrum of a small impurity of ${\mathrm{Gd}}^{3+}$ in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ has been examined at 24kMc/sec. The over-all zero-field splitting of the ground state of 1.24 ${\mathrm{cm}}^{-1\mathrm{}}$ is among the largest so far observed for ${\mathrm{Gd}}^{3+}$. The analysis of the spectra suggests that the ${\mathrm{Gd}}^{3+}$, which has twice the ionic radius of aluminum, essentially entered substitutionally for the ${\mathrm{Al}}^{3+}$ (but distorted the environment in such a way as to approach a condition of nine-fold oxygen coordination, the symmetry still remaining $C_3$). The substitution of ${\mathrm{Gd}}^{3+}$ for ${\mathrm{Al}}^{3+}$ whose ionic radius is half as large would indicate that at impurity levels less than 0.02% matching of ionic radii is not an all-important criterion for incorporation into the lattice.