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 (B_2q) 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.