Analysis of crystal-field effect on luminescence spectra of Mn4+ (3d3) ion-doped double perovskite La2ZnTiO6 phosphor by semiempirical computations: exchange charge model and superposition model†
Abstract
Herein, a systematic microscopic investigation of the crystal-field (CF) effects on Mn4+ ions doped at triclinic sites in the double perovskite La2ZnTiO6 crystal was performed using two independent modelling approaches based on CF theory, i.e., the exchange charge model (ECM) and the superposition model (SPM). The electronic energy levels of the Mn4+ ions were calculated using the available crystal structure data of pure La2ZnTiO6 host crystals and the photoluminescence spectra of the Mn4+-doped La2ZnTiO6 phosphor. A comparative analysis of the exchange charge effects and the influence of distortions of the Ti/MnO6 structure on the CF parameters (CFPs) was carried out. The strength of covalency and overlapping between the Mn4+ ion and its neighbours, as represented by the ECM parameter G, varies with the degree of triclinic distortion from the regular octahedral structure of the Ti/MnO6 polyhedra. The distortions and electrostatic interactions have a dominant impact on the 2nd-rank CFPs (B2q) and the CF invariants (S2) compared with those on their 4th-rank counterparts. It is shown that the 4th-rank CFPs and S4 are determined mainly by the exchange charge effects. The influence of Mn4+ impurity ions on the properties of doped phosphor crystals was also investigated. The methodology employed in this study may be useful for interpretation of the spectroscopic data of any transition metal ion in certain host lattices with low symmetry.