Defect study on Ce3+ in LiBaF3

Investigated are the electronic properties and the structure of a luminesce nce centre in LiBaF3 : Ce3+, consisting of a Ce3+ substitution at a Ba site , charge compensated by a Li+ substitution at a nearest Ba site. Using a pl ane wave pseudopotential molecular dynamics code, we determined the equilib rium structure of this defect complex, both for Ce3+ in the [Xe]4f(1) groun d state configuration, as well as for Ce3+ in its [Xe]5d(1) excited state c onfiguration. We performed quantum chemical cluster calculations at the Har tree-Fock level, to determine the energy of the 4f and 5d levels of Ce3+ in both these equilibrium structures. From these calculations we find the 4f --> 5d absorption lines, and the 5d(Z2) --> 4f emission line of Ce3+ in LiB aF3. From the Delta SCF between the 4f --> 5d(Z2) absorption line and the 5 d(Z2) --> 4f emission line we find a Stokes shift of 0.61 eV, compared to 1 eV found experimentally. The origin of this large Stokes shift is identifi ed as a strong coupling of the crystal-field splitting of the Ce3+ Sd-manif old with the displacement of four of its F nearest neighbours. (C) 2000 Els evier Science B.V. All rights reserved.