Impurities bound to vacancies in insulators: Electronic relaxation and physical properties of the Cr3+-V-M model center in KMF3 (M = Mg, Zn)

Article

关键词: HYPERFINE INTERACTIONS;    PARAMAGNETIC-RES;    SPIN-RESONANCE;    FLUORINE IONS;    KMGF3;    TRANSITION;    COMPLEXES;    EPR;    FLUORESCENCE;    SPECTROSCOPY;   

DOI  :  10.1103/PhysRevB.83.125123

来源: SCIE

【 摘 要 】

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 Cr3+ - V-M model center formed in KMF3 ( 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 M2+ vacancy, V-M, induces a significant structural relaxation on the CrF63- 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 Cr3+ - 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 CrF63- 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) -> T-4(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 CrF63- 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 Cr3+ - Li+ center formed in KMgF3 are also discussed for comparison.

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