Site-Dependent Luminescence and Thermal Stability of Eu²⁺ Doped Fluorophosphate toward White LEDs for Plant Growth

Eu²⁺ activated fluorophosphate Ba₃GdNa­(PO₄)₃F (BGNPF) with blue and red double-color emitting samples were prepared via a solid-state method in a reductive atmosphere. Their crystal structure and cationic sites were identified in light of X-ray diffraction pattern Rietveld refinement. Three different Ba²⁺ sites, coordinated by six O atoms referred to as Ba1, two F and five O atoms as Ba2, and two F and six O atoms as Ba3, were partially substituted by Eu²⁺. Photoluminescence emission (PL) and excitation (PLE) spectra of phosphor BGNPF:Eu²⁺ along with the lifetimes were characterized at the liquid helium temperature (LHT), which further confirm the existence of three Eu²⁺ emitting centers resulting in 436, 480, and 640 nm emission from the 5d → 4f transitions of Eu²⁺ in three different Ba²⁺ crystallographic sites. These emissions overlap with the absorption spectra of carotenoids and chlorophylls from plants, which could directly promote the photosynthesis. Temperature-dependent PL spectra were used to investigate the thermal stability of phosphor, which indicates that the PL intensity of BGNPF:0.9% Eu²⁺ with optimal composition at 150 °C still keeps 60% of its PL intensity at room temperature, in which blue emission has higher thermal-stability than the red emission. Furthermore, the approaching white LED devices have also been manufactured with a 365 nm n-UV LED chip and present phosphor, which make operators more comfortable than that of the plant growth purple emitting LEDs system composed of blue and red light. Results indicate that this phosphor is an attractive dual-responsive candidate phosphor in the application n-UV light-excited white LEDs for plant growth