Color Tuning of (K1−x,Nax)SrPO4:0.005Eu2+, yTb3+ Blue-Emitting Phosphors via Crystal Field Modulation and Energy Transfer

Two series of K_1-xNa_xSrPO₄:0.005Eu ²⁺ and K_0.4Na_0.6Sr_0.995-yPO ₄:0.005Eu²⁺, yTb³⁺ phosphors are synthesized via a high-temperature solid-state reaction. Their emission color can be tuned from deep blue to blue-green by modulating the crystal field strength and energy transfer. Partial substitution of K⁺ with Na⁺ results in a contraction and distortion of the unit cell of the K_1-xNa _xSr_0.995PO₄:0.005Eu²⁺ host, tuning the emission from 426 to 498 nm. The red-shifted emission is attributed to an increased crystal field splitting for Eu²⁺ in a lowered symmetry crystal field. The tunable emission is further demonstrated in the cathodoluminescence spectra, which indicates that the luminescence distribution of the K_1-xNa_xSr_0.995PO₄:0. 005Eu²⁺ phosphor is very homogenous. Additionally, utilizing the principle of energy transfer, the emission color can be further tuned by co-doping with Tb³⁺. The chromaticity coordinates for the co-doped phosphor, K_0.4Na_0.6Sr_0.995-yPO ₄:0.005Eu²⁺, yTb³⁺, can be adjusted from (0.202, 0.406) for y = 0 to (0.232, 0.420) for y = 0.09. The energy transfer processes from the sensitizer (Eu²⁺) to the activator (Tb ³⁺) are studied and demonstrated to have a resonance-type dipole-dipole interaction mechanism, with the critical distance of the energy transfer calculated to be 12.46 Å using a concentration quenching method.