Anomalous superhyperfine tensor observed in BaFCl doped with the 5 d1 ion La2+: Role of 5d-4f hybridization

Abstract

The fluorine superhyperfine (shf) tensor measured in BaFCl: La2+ has been found to be practically isotropic, a result which is certainly anomalous when compared to that for d9 centers with one unpaired electron in a x2 - y2 orbital. This puzzling fact has been explored by means of density functional calculations. Obtained results confirm that in the C4v equilibrium geometry the unpaired electron lies in a b1 (∼ x2 - y2) 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 D4h and C4v La F4 2-, Y F4 2-, and Ag F4 2- centers have also been investigated. Although the obtained results stress the high dependence of the isotropic shf constant As on the metal-ligand distance R, a near isotropy of the shf tensor is only reached for La F4 2- (but not for Y F4 2-) under C4v symmetry which corresponds to the actual symmetry of the La2+ center in the BaFCl lattice. The origin of this peculiar situation is shown to come from the mixing between 5d and 4f orbitals of La2+ allowed in C4v symmetry thus stressing the role played by 4f orbitals in bonding properties. Writing As =C R- ns it is shown that for the D4h La F4 2- and Y F4 2- complexes the exponent ns is around 20, while it is only equal to 4 for Ag F4 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 d1 centers and the d9 Ag F4 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: La2+ is indeed a 5d→4f transition. © 2007 The American Physical Society.