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21-ion cluster used for calculations of the FeOF 5 4 # complex in KMgF 3 .
Source publication
. The equilibrium geometry of the C
4ν FeOF5
4− center in KMgF3 has been explored by means of density functional theory calculations. Particular attention has been paid to the changes of
electronic density on passing from the O
h
FeF6
3− center in KMgF3 to FeOF5
4−, as well as to the variations undergone by the isotropic superhyperfine constant of...
Contexts in source publication
Context 1
... systems chosen for exploring the reliability of the transferability postu- late are the O h FeF 6 3# center in KMgF 3 [14] and the C 4v FeOF 5 4# center formed in the same lattice [15]. According to Fig. 1 3 comes from the equatorial and axial fluorine nuclei of the complex [15]. As a salient feature, a huge difference between the equatorial, A s (F eq ) " 72.3 MHz, and axial, A s (F ax ) " 30.8 MHz, isotropic shf constants has been measured experi- mentally ...
Context 2
... have been performed in the framework of the DFT by means of the Amsterdam density functional code [17]. All results shown in this work on the Fe 3! O 2# pair in KMgF 3 have been performed on the 21-atom FeF 4 OFK 8 Mg 6 16! cluster depicted in Fig. 1. The exchange-correlation energy was computed accord- ing to the generalized gradient approximation by means of the PerdewBurke Ernzerhof functional [18]. All atoms were described through basis sets of TZP quality (triple-z Slater-type orbitals plus one polarization function) given in the program data base and the core electrons ...
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Citations
... 4,5 Also, a density functional theory (DFT) study was carried out for the equilibrium geometry of this center as well as for the changes in electronic density. 6 Another study was devoted for TR center by means of complete diagonalization method. 7 Previously, we have investigated the local structure around Cr 3þ centers in perovskite KMgF 3 crystal and have shown that ab-initio calculations (DFT), in addition to their various profits on determination of the properties related with quantum chemistry, can be effectively performed to provide the local structural parameters around a paramagnetic dopant ion. ...
Analyses of the local crystal and electronic structure in the vicinity of Fe3+ centers in perovskite KMgF3 crystal have been carried out in a comprehensive manner. A combination of density functional theory
(DFT) and a semi-empirical superposition model (SPM) is used for a complete analysis of all Fe3+ centers in this study for the first time. Some quantitative information has been derived from the DFT calculations on both the electronic structure and the local geometry around Fe3+ centers. All of the trigonal (K-vacancy case, K-Li substitution case, and normal trigonal Fe3+ center case), FeF5O cluster, and tetragonal (Mg-vacancy and Mg-Li substitution cases) centers have been taken into account based on the previously suggested experimental and theoretical inferences. The collaboration between the experimental data and the results of both DFT and SPM calculations provides us to understand most probable structural model for Fe3+ centers in KMgF3.
... An ab initio study carried out on both the FeF 3− 6 (O h symmetry) and the C 4v FeF 5 O 4− centers formed in KMF 3 fluorides (M = Mg, Zn) demonstrated that changes on the electronic density of F − ions due to the presence of an O 2− ligand cannot be neglected at all. 5,21 Seeking to improve our knowledge on complex I-V centers in insulating materials, we want to explore the model center Cr 3+ − V M formed in cubic fluoroperovskites such as KMF 3 (M = Mg, Zn) doped with Cr 3+ . 11,[22][23][24][25][26] In that center, a close M 2+ vacancy, V M , is bound to the impurity, as shown in Fig. 1. ...
... To our knowledge, no clear experimental information on the superhyperfine tensor for the Fe 3+ − V M center in fluorides has been reported, but there are good EPR data on the FeF 5 O 4− centers formed in KMF 3 fluorides (M = Mg, Zn) demonstrating a significant reduction of f s in the axial ligand due to the presence of O 2− . 5,21,69,70 Preliminary calculations on the Fe 3+ − V M center in KMgF 3 show the existence of an important increase of f σ induced by the vacancy in the closest ligand. Further work along these lines is now underway. ...
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+-VM 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, VM, 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 VM forces all the ligands to move away, the Cr3+-F- distance corresponding to the furthest ligand, Ffar, 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 VM on the CrF63- complex, causing a charge of 0.2e to be transferred from the closest ligand to VM, Fnext, mainly to Ffar, and, to a lesser extent, to any equatorial ligand. This transfer of charge is mainly accomplished through orbitals lying in planes containing the C4 axis. In spite of these changes due to the vacancy, the 4A2g→4T2g 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 VM. In particular, the present study explains that the dominant component of the superhyperfine tensor for the Fnext ligand is clearly higher than that for Ffar, 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.
... Among the recently published theoretical works on this subject we mention the first-principles-based analysis of the 10Dq and charge transfer transitions for the Cr 3+ in Cs 2 NaYF 6 , Cs 2 NaYCl 6 and Cs 2 NaYBr 6 [16], pressure effects for the Cr 3+ ions in Cs 2 NaScCl 6 and Cs 2 NaYCl 6 [17], determination of the different origin of colors of MgAl 2 O 4 and Be 3 Si 6 Al 2 O 18 doped with Cr 3+ [18], consideration of properties of the chemical bonds between Fe 3+ and ligands in KMgF 3 [19], etc. In all these publications the DFT-based ab initio methods were successfully applied. ...
A detailed microscopic study of the crystal field strength 10Dq for different interionic distances in cubic SrTiO3 doped with three isoelectronic ions (Cr3+, Mn4+ and Fe5+) was performed. The exchange charge model of the crystal field was used to calculate the 10Dq values at different distances between impurity ions and ligands. The obtained results were represented by the power laws 1/Rn, with n = 4.9050, 5.7990 and 6.5497 for Cr3+, Mn4+ and Fe5+, respectively. For the first time the role of two different contributions (the point charge and exchange charge) into the total crystal field strength was studied separately. With the obtained 10Dq(R) dependences, a number of important physical quantities describing the optical and dynamical properties of impurity centers (such as the constants of the electron-vibrational interaction, Huang–Rhys parameters, Stokes' shifts, Jahn–Teller stabilization energies, changes of the chemical bond lengths due to the combined effect of the local vibrational normal modes, bulk modulus and Grüneisen constants for the a1g normal mode) were calculated. The obtained results are in good agreement with available experimental data and can be readily applied for analysis of the optical spectra, electron-vibrational interaction and pressure effects for these and other similar systems.
Allowing high transition metal (TM) ions doping concentrations makes the fluoroelpasolite crystals Cs2NaMF6 (M = Al, Ga) quite interesting in various laser applications. In this study, we explored the influence of the dopant ions on the electronic and structural properties of Cs2NaAlF6. Particular attention is paid to the influence of trivalent TM ions (Cr³⁺ and Fe³⁺) on the local structure around the possible substitutional cation sites by investigating the optimized geometry around impurity sites due to Cr³⁺ and Fe³⁺ using density functional theory (DFT). The obtained structural data are utilized for the calculations of zero-field splitting (ZFS) parameters (ZFSPs) and crystal field parameters (CFPs) of both dopant ions independently in the frame of semi empirical superposition model (SPM). It is shown that remarkable change on the ZFSPs comes not from the distortion on metal-ligand distance but from that on the angular positions of the ligands around the impurity centers. It is clearly shown that the substitution of both Cr³⁺ and Fe³⁺ ions take place with significant influence on electronic structure and geometry for the trivalent Al³⁺ sites. The presented results here have significant potential to better understand the low-symmetry effects in especially optical applications of Cs2NaAlF6:Cr³⁺ and Cs2NaYF6:Fe³⁺ systems. Particularly, the calculated CFPs for Cs2NaYF6:Fe³⁺ system may serve as the only record in literature.
In the second chapter of the book the authors present the results of theoretical studies ofthe energy levels schemes of all 3dn (n=1, 9) ionsin various crystals at the substitutionalsites. Systematic calculations are described in all details; they include the overlap integrals between the impurityions' and ligands' wave functions; the crystal field parameters calculations, and diagonalization of the crystal field Hamiltonians for each considered case. The calculated results arediscussed and compared with experimental data and with similar results from literature. The chapter also contains a comprehensive literature review on the properties of 3d-ions doped crystals. © Tsinghua University Press, Beijing and Springer-Verlag Berlin Heidelberg 2013. All rights are reserved.
