Anomalous superhyperfine tensor observed in BaFCl doped with the5d1ionLa2+: Role of

J. M. Garcı́a-Lastra

¹

,

Hans Bill

²

,

M. T. Barriuso

³

et al.

Abstract

The fluorine superhyperfine ͑shf͒ tensor measured in BaFCl: La 2+ has been found to be practically isotropic, a result which is certainly anomalous when compared to that for d 9 centers with one unpaired electron in a x 2 − y 2 orbital. This puzzling fact has been explored by means of density functional calculations. Obtained results confirm that in the C 4v equilibrium geometry the unpaired electron lies in a b 1 ͑ϳx 2 − y 2 ͒ orbital which overlaps with the 2s orbitals of four F − ligands. For explaining the origin of the near isotropy, which is well reproduced by the present calculations, the simple D 4h and C 4v LaF 4 2− , YF 4 2− , and AgF 4 2− centers have also been investigated. Although the obtained results stress the high dependence of the isotropic shf constant A s on the metal-ligand distance R, a near isotropy of the shf tensor is only reached for LaF 4 2− ͑but not for YF 4 2− ͒ under C 4v symmetry which corresponds to the actual symmetry of the La 2+ center in the BaFCl lattice. The origin of this peculiar situation is shown to come from the mixing between 5d and 4f orbitals of La 2+ allowed in C 4v symmetry thus stressing the role played by 4f orbitals in bonding properties. Writing A s = CR −n s it is shown that for the D 4h LaF 4 2− and YF 4 2− complexes the exponent n s is around 20, while it is only equal to 4for AgF 4 2− . This huge difference is shown to stem from the quite distinct slope of the radial d wave function at the equilibrium distance for the two d 1 centers and the d 9 AgF 4 2− unit. Finally, the present calculations strongly support that the intense band peaked at 17 890 cm −1 recorded in the optical absorption spectrum of BaFCl: La 2+ is indeed a 5d → 4f transition.

Instability of some divalent rare earth ions and photochromic effect

“…Direct and inverse optical transformations of divalent lanthanum (La 2+ ) ions into F centers have been studied in BaFCl-La and SrFCl-La crystals by optical means and EPR. Although the explanation of the authors of [39,40] differs from our interpretation, the similarities between the photo-induced electron transition from the divalent ion to the anion vacancy are worthy of attention.…”

Spectrochemical Series and the Dependence of Racah and 10Dq Parameters on the Metal−Ligand Distance: Microscopic Origin

“…In fact, while the exponent n s is found to be equal to 7.7 for CrF 6 3− , the calculated value for CrI 6 3− is about 40% higher. It is worth noting that n s values around 20 have been calculated for LaF 4 2− complexes involving the unusual La 2+ cation . This conclusion is supported by EPR data on CaF 2 :La 2+ showing that the superhyperfine lines are extremely sensitive to the application of an uniaxial stress …”

“…76 This conclusion is supported by EPR data on CaF 2 :La 2þ showing that the superhyperfine lines are extremely sensitive to the application of an uniaxial stress. 76 4.3. Contributions to 10Dq and the Spectrochemical Series.…”

“…It is worth noting that n s values around 20 have been calculated for LaF 4 2complexes involving the unusual La 2þ cation. 76 This conclusion is supported by EPR data on CaF 2 :La 2þ showing that the superhyperfine lines are extremely sensitive to the application of an uniaxial stress. 76 4.3.…”