Crystal-Field Spectra of d3,7 Ions. V. Tetrahedral Co2+ in ZnAl2O4 Spinel

Absorption and fluorescence spectra of Co²⁺ in the tetrahedral ${\mathrm{(T}}_d)$ site of ZnAl₂O₄ spinel (gahnite) at low temperatures have been observed and analyzed. The resulting energy‐level diagram includes four quartet states and nine doublets, and has been interpreted first in terms of a weak field formalism without spin–orbit coupling. In this analysis the “free‐ion” levels for $D_q\text{\hspace{0.17em}=\hspace{0.17em}0}$ were deduced and compared with those for a free ion outside the influence of the crystalline environment. The reduction in the electrostatic interaction parameters $F_2$ and $F_4$ in the crystal is due to the effect of covalency. A second approach has been used to analyze the fine structure of the energy levels in terms of a strong field spin–orbit formalism. The fine structure can be explained for all the bands on the basis of simple spin–orbit splitting, but the possibility of a Jahn–Teller distortion of the ${}^4{T}_{\text{1a}}$ state cannot be entirely eliminated.