The multiplet structures of tetrahedrally coordinated Cr4+ in
the three silicate crystals Mg2SiO4 (forsterite),
Ca2MgSi2O7 (åkermanite), and Y2SiO5 (yttrium
orthosilicate (YSO)) were calculated by the many-electron
electronic structure calculation method developed by the authors.
The method is a hybrid of the molecular orbital method
based on the density functional theory and the configuration
interaction approach. For every crystal, the calculations were
conducted by using cluster models with the three sizes: (A)
(CrO4)4- (without point charges) models, (B)
(CrO4)4- (with point charges) models, and (C)
(CrMg9Si2O37)44- (forsterite),
(CrCa6Mg2SiO38)52- (åkermanite), and
(CrY8O37)46- (YSO) models. The calculated multiplet
energies of the triplet states agreed with the experimentally
obtained peak energies in the absorption spectra in the literature.
The theoretical spectra showed polarization dependence of the
peak intensity. The best agreement was found in the results
obtained from the largest models C. The difference in
polarization dependence between Cr4+:forsterite and
Cr4+:åkermanite was related to the different mixing of the
many-electron wave functions as regards the 3T2(et2) and
3T1(et2) triplet terms. The covalency of the impurity-level
molecular orbitals was also analysed. The results of models C
indicated that the wave functions of the atoms outside the CrO4
tetrahedron should not be neglected. Both the degree of covalency
and the correlation-correction factor, which was introduced in the
method, were regarded as reduction factors of two-electron
repulsion. The two factors were multiplied together, and the
reduction factor was a convenient indicator for simply
evaluating the magnitude of the reduction. The traditional
nephelauxetic parameter was obtained as 0.49. Some
empirical values given recently in the literature were confirmed to
have appropriate magnitude.