MgAl2O4:Cr3+ and emerald display a different colour but the local symmetry is the same: Microscopic origin

The difference in colour between emerald (Be3Si6Al2O18:Cr3+, green) and the Cr3+-doped spinel MgAl2O4 (red) is striking, considering that in both systems colour is due to CrO69- complexes with D3 symmetry and the measured Cr3+-O2- distance is practically the same (1.98 and 1.97 angstroms, respectively). By means of density functional calculations it is shown that this surprising difference can reasonably be explained once the electric field, ER, which all lattice ions lying outside the CrO69- complex exert on localized electrons, is taken into consideration. The origin of the different shape of ER in the two host lattices is analysed in detail. It is shown that ER raises (decreases) the 2p(O) levels for Be3Si6Al2O18:Cr3+ (MgAl2O4:Cr3+) along the trigonal axis thus favouring a decrease (increase) of 10Dq. The present work demonstrates the key role played by ER (not considered in the traditional ligand field theory) for understanding the differences exhibited by the same complex embedded in host lattices which do not have the same crystal structure. Some remarks on the colour of Cr2O3 pure compound are also reported.