Optically Observed Exchange Splittings in Antiferromagnetic Cr2O3

Well‐defined exchange splittings of the R lines of Cr³⁺ in Cr₂O₃ have been observed in absorption at low temperatures. The majority of the transitions originate on the lowest ${\mathrm{(M}}_s\mathrm{=-}\frac{3}{2})$ component of the ⁴A₂ (ground) state and terminate on the four (exchange split) components of the two ²E levels. The lines show strong polarization effects, the outer two components of the four line group appearing in the perpendicular spectrum $\mathrm{(E\perp c\hspace{0.17em}}\mathrm{axis})$ and the inner two components appearing in the parallel spectrum. From the observed polarizations and from relative intensity measurements the identity of each of the components has been deduced. On the basis of the identifications made, the exchange splitting of the R₁ doublet is approximately 183 cm⁻¹ and that of the R₂ doublet is approximately 139 cm⁻¹. A ground‐state splitting of 245±20 cm⁻¹ is obtained from a weak line originating on the $M_s\mathrm{=-}\frac{1}{2}{\mathrm{\hspace{0.17em}(}}^4{A}_2)$ level. The trigonal field (plus spin orbit) splitting of the ²E levels can also be deduced from the spectrum. The splitting would appear to be very small (of the order of 1 cm⁻¹) compared with that of Cr³⁺ in ruby (29.6 cm⁻¹) and of the opposite sign.