MS-Xα and extended huckel calculations on lif : Ni⁺

¹ Departamento de Física Moderna, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain
² D.C.I.T.T.Y.M., Sección Ciencia de Materiales, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain

Abstract

Selfconsistent charge Extended Huckel calculations on clusters including up to 81 atoms as well as MS-Xα calculations on a 27 atoms cluster have been performed in order to investigate theoretically a good number of E.P.R. and optical parameters for LiF:Ni⁺ with elongated octahedron symmetry around Ni+. In agreement with experimental data the present results give f_σ close to 1%, xy→x²-y² excitations close to 6000 cm^-1 while the charge transfer transitions lie in the vacuum ultraviolet region. The mainly 3d levels lie in the gap about 4 eV above the top of the valence band while the mainly 4p levels lie about 2 eV below the bottom of the conduction band. The levels mainly built from 2 p(F') of four ligands are also strongly localized in the distorted NiF65- complex but located not on the top of the valence band but about 3.5 eV below. When the "equatorial" Ni⁺-F^- distance, R_eq, decreases a) the energy of charge-transfer and crystal-field excitations increases, b) f_s increases much more rapidly than f_σ, although √f_s/S_s is practically constant, independent of R_eq. This fact supports the use of the isotropic superhyperfine constant for measuring the true R_eq value.

Résumé

De nombreaux paramètres RPE et optiques ont été calculés sur un agrégat représentatif de l'impureté Ni⁺ dans LiF. Deux méthodes ont été utilisées, MSXalpha appliquée à un agrégat de 27 atomes, et EHT avec itérations de charges appliquée à des agrégats de 7, 27, et 81 atomes. La symétrie retenue est D4h sur le site Ni⁺, et l'influence de la distance équatoriale sur le déplacement des bandes optiques et sur les paramètres RPE a été étudiée.