  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.
