Observation ofCr4+inα−Al2O3

The paramagnetic resonance spectrum of single crystals of $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ containing chromium grown under conditions favoring the stabilization of ${\mathrm{Cr}}^{4+}$ has been observed. Data were taken at $X$ band at liquid-helium temperatures. A single absorption line was observed which is attributed to a $\Delta M=\pm 2$ transition. The crystal-field splitting parameter is inferred from the temperature dependence of the strength of the resonance to be $D\cong +7\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The spectroscopic splitting factor is $g_{\parallel }={1.90}_{-0.01\mathrm{}}^{+0.02\mathrm{}}$. A distribution of lower symmetry fields gives rise to an asymmetric line; the range of $E$ values deduced from this asymmetry is $0\lesssim E\lesssim 0.05$ ${\mathrm{cm}}^{-1\mathrm{}}$, where $2E$ is the zero-field doublet splitting. Optical absorption data and radiation effects are presented.