Ab initio simulation of photoluminescence: Bi3+ in Y2O3 (S6 site)

Abstract The aim of this paper is to present the methodology of ab initio simulation in the field of photoluminescent materials using the local embedded cluster technique. In our approach the effect of the crystalline environment is introduced via ab initio embedding model potentials and the resulting hamiltonian of the perturbed cluster is treated using state-of-the-art quantum chemistry methods including a refined spin–orbit coupling treatment. The method is applied to the case study of a Bi 3+ ion embedded in a site of point symmetry S 6 of Y 2 O 3 sesquioxide. Comparison of the results obtained with two kinds of embedded clusters (a monoatomic Bi 3+ cluster and a more realistic molecular (BiO 6 ) 9− cluster) makes it possible to separate the respective parts of crystal field and molecular bonding. Both effects contribute to a marked stabilization of Bi 3+ 6s6p z versus Bi 3+ 6s6p x,y , which causes a spectacular quenching of spin–orbit effects. The strong decrease of the 6s 2 -6s6p z energy separation (10 → 4 eV) observed on embedding Bi 3+ is entirely due to the small covalent character of the mainly ionic Bi–O bond, not at all to crystal field. The lowest-lying excited states calculated in the range 4.3–5.1 eV are found to be the four components of the Bi 3+ (6s6p z ) 3 A u and 1 A u complex. The next states, above 6.9 eV, are charge transfer states corresponding to the excitation of an electron of the valence 2p band of oxygen of the ground state into the 6p z bismuth orbital. In the qualitative orbital model this allows to deduce an estimate of the position of the Bi 3+ ground state with respect to the top of the oxygen valence band: 2.6 eV.