The origin of the spectrochemical series and the different dependence of crystal-field splitting (10Dq) and Racah parameters on the metal-ligand distance, R, is explored through ab initio calculations on Cr(3+)-doped K2NaScF6, Cs2NaYCl6, Cs2NaYBr6, and Cs2NaYI6 lattices. For this purpose both periodic and cluster calculations have been performed. An analysis of ab initio results proves that 10Dq values mostly come from the small admixture of deep nLs ligand orbitals present in the antibonding eg(∼ x(2)-y(2),3z(2)-r(2)) level and not from the dominant covalency with valence nLp ligand orbitals, which is actually responsible for the reduction of Racah parameters. This study thus reveals the microscopic origin of the stronger dependence upon R of 10Dq when compared to that observed for Racah parameters, thus explaining why electronic transitions which are 10Dq-independent give rise to sharp optical bands. As a salient feature, while the covalency with nLp levels increases significantly on passing from CrF6(3-) to CrI6(3-), the nLs admixture in eg is found to be practically unmodified. This fact helps to understand the progressive decrease of 10Dq through the series of CrF6(3-), CrCl6(3-), CrBr6(3-), and CrI6(3-) complexes embedded in the corresponding host lattices when compared at the corresponding equilibrium distance at zero pressure. The growing importance of the nLs admixture is well-depicted using deformation density diagrams on passing from the ground state (4)A2(t2g(3)) to the (4)T2(t2g(2)eg) excited state depicted at several R values.
The magnitude of tunneling splitting, 3Gamma, that governs decoherence in quantum systems under symmetric potentials, is frequently unknown. Using first-principles calculations, we have obtained 3Gamma for a number of E[symbol:see text]e Jahn-Teller impurities in solids. Calculated values span 6 orders of magnitude on passing from KCl:Ag{2+} to MgO:Cu{2+}. Values associated with Cu2+- and Ag2+-doped MgO are 2 orders of magnitude larger than those previously assumed and consistent with the existence of a dynamic Jahn-Teller effect. The origin and enhancement with pressure of these high 3Gamma values is discussed in detail.
This work is aimed at gaining a better insight into the influence of a close vacancy, V, on the properties of impurities in insulating materials. To achieve this goal the Cr 3+ − V M model center formed in KMF 3 (M = Mg, Zn) fluoroperovskites has been explored in detail by means of ab initio calculations on clusters involving up to 87 ions. It is shown that the presence of the M 2+ vacancy, V M , induces a significant structural relaxation on the CrF 3− 6 cubic complex which cannot be fully understood assuming that ions were rigid spheres that could not be polarized. Thus, although V M forces all the ligands to move away, the Cr 3+ − F − distance corresponding to the furthest ligand, F far , is found to be slightly higher than that for the closer equatorial ions. This unexpected fact is shown to be due to the electronic relaxation also induced by V M on the CrF 3− 6 complex, causing a charge of 0.2e to be transferred from the closest ligand to V M , F next , mainly to F far , and, to a lesser extent, to any equatorial ligand. This transfer of charge is mainly accomplished through orbitals lying in planes containing the C 4 axis. In spite of these changes due to the vacancy, the 4 A 2g → 4 T 2g optical transition is found to be weakly altered, a fact that concurs with available experimental data and whose origin is discussed. In contrast, electron paramagnetic resonance parameters such as gyromagnetic or superhyperfine tensors, which do depend on the electronic density around a point of the CrF 3− 6 complex, are shown to be particularly sensitive to the electronic relaxation induced by V M . In particular, the present study explains that the dominant component of the superhyperfine tensor for the F next ligand is clearly higher than that for F far , in agreement with experimental data. The relevance of the present results for understanding the electronic properties of other systems involving vacancies is also discussed. Some results on the Cr 3+ − Li + center formed in KMgF 3 are also discussed for comparison.
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