This work
is aimed at clarifying the changes on optical spectra of Cr3+
impurities due to either a host lattice variation or a hydrostatic pressure,
which can hardly be understood by means of the usual Tanabe–Sugano (TS) approach assuming that the
Racah parameter, B, grows when covalency decreases. For achieving this
goal, the optical properties of Cr3+-doped LiBaF3 and
KMgF3 model systems have been explored by means of high level ab
initio calculations on CrF63– units subject to the
electric field, ER(r), created by the rest of the
lattice ions. These calculations, which reproduce available experimental data,
indicate that the energy, E(2E), of the 2E(t2g3)
→ 4A2(t2g3) emission transition is
nearly independent of the host lattice. By contrast, the energy difference
corresponding to 4A2(t2g3) → 4T1(t2g2eg1)
and 4A2(t2g3) → 4T2(t2g2eg1)
excitations, Δ(4T1;
4T2), is shown to increase on passing from the normal to
the inverted perovskite host lattice despite the increase in covalency, a fact
which cannot be accounted for through the usual TS model. Similarly, when the
Cr3+–F– distance, R, is reduced both Δ(4T1; 4T2)
and the covalency are found to increase. By analyzing the limitations of the
usual model, we found surprising results that are shown to arise from the
deformation of both 3d(Cr) and ligand orbitals in the antibonding eg
orbital, which has a σ
character and is more extended than the π t2g orbital. By contrast, because
of the higher stiffness of the t2g orbital, the dependence of E(2E)
with R basically follows the corresponding variation of covalency in
that level. Bearing in mind the similarities of the optical properties
displayed by Cr3+ impurities in oxides and fluorides, the present
results can be useful for understanding experimental data on Cr3+-based
gemstones where the local symmetry is lower than cubic.
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