Cu3+Ion in Corundum

Abstract

The paramagnetic resonance spectrum of the ion ${\mathrm{Cu}}^{3+}$ (configuration $3d^8$ which is isoelectronic with ${\mathrm{Ni}}^{2+}$) in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ has been studied. The results are correlated with the observed optical absorption spectrum in terms of crystal field theory. In addition, precise information about the electron-nuclear interactions of this ion was obtained by means of the electron nuclear double resonance (ENDOR) technique. The ${\mathrm{Cu}}^{3+}$ entered the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ lattice as a substitutional impurity for ${\mathrm{Al}}^{3+}$. The ion exhibits a spectrum typical of a center with $S=1\mathrm{}$ in an axially symmetrical site. Each electronic transition has four hyperfine components arising from a nuclear spin of $\frac{3}{2}$. The lines corresponding to the two isotopes ${\mathrm{Cu}}^{63}$ and ${\mathrm{Cu}}^{65}$ were unresolved in the paramagnetic spectrum. The observed transitions yielded the following parameters: $D=-0.18838\mathrm{}\pm 0.00004$ ${\mathrm{cm}}^{-1\mathrm{}}$, $g_{\mathrm{II}}=2.0784\mathrm{}\pm 0.0005$, and $g_{\perp }=2.0772\mathrm{}\pm 0.0005$. In ENDOR experiments at 1.3°K, $\Delta m_s=0\mathrm{}$, $\Delta m_I=\pm 1$ transitions were observed with linewidths of about 50 kc/sec. These could be fitted satisfactorily only by postulating an effective field at the nucleus equal to $(1+\delta ) H_0$, where $H_0$ is the external field and $\delta =0.0087\mathrm{}$. Other interaction parameters are $A_{63}=-192.947\mathrm{}\pm 0.001$ Mc/sec; $A_{65}=-206.679\mathrm{}\pm 0.001$ Mc/sec; $B_{63}=-180.10\mathrm{}\pm 0.05$; $B_{65}=-192.916\mathrm{}\pm 0.005$; $e^{2}q{Q}_{63}=-0.168\mathrm{}\pm 0.004$ Mc/sec. From the magnitude of $\delta$, an estimate is made of $〈\frac{1}{r^3}〉$ of ${\mathrm{Cu}}^{3+}$. The surprisingly small value of $e^2\mathrm{qQ}$ is also discussed. The hyperfine structure anomaly $\left(\frac{A_{63}}{A_{65}}\right)\left(\frac{g_{65}}{g_{63}}\right)-1=(0.0145\mathrm{}\pm 0.0020)\%$.%. Since this agrees with the value obtained for $s_{\frac{1}{2}}$ electrons in Cu as measured by atomic beam magnetic resonance, one may conclude that the hyperfine interaction in ${\mathrm{Cu}}^{3+}$ has its major origin in $s_{\frac{1}{2}}$ electrons polarized by $s-d$ interactions.