The synergy effect of rare earth cations on local structure and PL emission in a Ce3+:REPO4 (RE = La, Gd, Lu, Y) system

Abstract

REPO₄ (RE = La, Gd, Lu, Y) serves as an excellent host lattice due to its stable physicochemical properties and optical inertia. Doping Gd³⁺(La³⁺) into LuPO₄ can form mixed crystals, increasing the Gd³⁺(La³⁺) concentration will induce the phase transition from tetragonal to hexagonal lattices, and the variation of the local structure around the Ce³⁺ activator will influence its 5d-level position and consequently 5d → 4f radiation transition. This can be attributed to the synergy effect of rare earth ions in REPO₄, however, the essential mechanism of such a synergy effect on the local structure and optical property is still poorly understood. Here, we study the synergy effect of rare earth ions on the phase transition and PL emission in a Ce³⁺:REPO₄ system on the basis of the relationship between the composition-dependent local structure around Ce³⁺ and its PL emission properties from a molecular view. The competition between Lu³⁺ and Gd³⁺(La³⁺) in REPO₄ not only influences the relative atomic position but also varies the symmetry of anion groups. Infrared absorption bands indicate that the activation of P−O bonding promotes phase transition and enhances PL emission intensity. The PL emission intensity of Ce³⁺ is higher in a REPO₄ host with a lower site symmetry PO₄³⁻ group (C₂) than that with a higher site symmetry PO₄³⁻ group (D_2d). An increased disorder degree in Ce:Gd_xLu_1−xPO₄ mixed crystals leads to the shift of the 5d-level of Ce³⁺ towards a higher position, resulting in the blue shift of the PL emission wavelength. Moreover, the 5d → 4f emission of Ce³⁺ may also be modulated towards a larger wavelength via substituting the cation site with larger-radius cations under a particular crystallographic structure in REPO₄. Our results highlight the importance of disordered local structures as well as activated anion groups in the enhanced PL emission of Ce³⁺ activators in a host lattice.