Theoretical Investigations of the Zero-Field Splittings for Cr³⁺ and Cr⁴⁺ in Al₂O₃ Crystal

Based on the crystal-field theory,the optical absorption spectra and zero-field splittings（ZFS） D for Cr³⁺ and Cr⁴⁺ in the Al₂O₃ crystal have been studed.The theoretical results agree well with experimental findings.The investigations show that the local structure of the （CrO₆）^8- group in the Al₂O₃:Cr⁴⁺ crystal is of the compressed trigonal distortion（θ_k-θ_ko ≈3°）.That is why the ZFS D （≈7cm^-1）for the Al₂O₃:Cr⁴⁺ is unusually large.