Paramagnetic Resonance and Optical Absorption Spectra of Cr3+ in MgO

The paramagnetic resonance spectrum of the trivalent chromium ion has been detected at a wavelength of 1.2 cm. The spectrum is isotropic with one line at $g=1.9800\ifmmode\pm\else\textpm\fi{}0.0006$. The hyperfine structure of the ${\mathrm{Cr}}^{53}$ isotope in its natural abundance was detected. The hyperfine structure constant $A=(1.60\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ permits an evaluation of the nuclear magnetic moment as 0.475 nm. The narrow line and the failure to observe any ground state splitting show that the trivalent chromium ion is exposed to a very pure cubic field. There is no indication of associated vacancies with the ${\mathrm{Cr}}^{3+}$ ion. Optical absorption lines were found at 16 000, 22 700, and 29 700 ${\mathrm{cm}}^{\ensuremath{-}1}$ as well as a beginning of absorption at 46 000 ${\mathrm{cm}}^{\ensuremath{-}1}$. The 16 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ line is caused either by a quartet-doublet transition or by the ${\mathrm{Cr}}^{2+}$ ion. The other absorption lines can be fitted with $Dq=2270$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and the $^{4}P$ state at 10 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ above the $^{4}F$ state. An analysis of the paramagnetic and optical absorption spectra indicate very strong crystal field and some covalent bonding. Probably the ${\mathrm{Cr}}^{3+}$ and the surrounding six oxygen atoms have to be regarded as a very tightly knit unit.