Analysis of crystal-field effects on the energy levels of Mn4+ ions doped in different photoluminescent host lattices, exhibiting lowering of site symmetry: Exchange charge and superposition model calculations, for potential applications

Abstract

Herein, we presented a comparative analysis of crystal-field (CF) effects on the energy levels of Mn4+ ions doped in the various host lattices exhibiting site symmetry from: triclinic: Y2MgTiO6, La2MgTiO6, Ca2YTaO6, Ca2YSbO6; trigonal: Ba2LaSbO6, LaTiSbO6; tetragonal: Ba2GdTaO6; to octahedral: Li2MgTiO4, Ba2GdSbO6, Ba2YSbO6. For modelling CF parameters (CFPs), we employed exchange charge model (ECM) using one model parameter (G), and superposition model (SPM) using four model parameters. A modified crystallographic axis system (CAS*) was adopted in ECM and SPM calculations for low symmetry hosts. The modelled CFPs served as input for crystal-field analysis/microscopic spin-Hamiltonian program to calculate the Mn4+ energy levels and match them with photoluminescence spectra of Mn4+-doped crystals. The 2nd-rank ECM/CFPs vary with G depending on distortions of the metal (M)-ligand (L) MLn polyhedron. The 4th-rank ECM/CFPs depend predominantly on the exchange charge effects. The ECM yields seemingly a better agreement between the observed and calculated energies than the SPM one. The disparity between the CFP sets calculated using SPM and ECM approaches in CAS*, while yielding same CFP strength, poses dilemmas. To solve these dilemmas, we employed the triclinic standardization of CFP sets, thus uncovering intricate low symmetry aspects inherent in ECM/CFPs, hitherto not realized. This method proved very useful for meaningful analysis and interpretation of CFP sets. Our CFP modelling illustrates importance of well-defined axis systems and adequate treatment of the actual triclinic (and monoclinic) site symmetry of dopant ions, instead of octahedral symmetry (usually adopted by experimentalists), in modelling of CFPs for triclinic symmetry cases. The relations between the Racah parameters, B and C, and 2Eg → 4A2g peak emission energy of Mn4+ ions are extended and suitably modified for triclinic symmetry. These modifications may be significant in engineering the local CF environment by suitably choosing the host crystals and dopant ions to improve the luminescence properties of phosphors for practical applications. The influence of dopant metal ions on the local structure and thus spectroscopic properties of doped crystals was also explored.