Color tuning of (K_(1-x)Na_x)SrPO4:0.005Eu~(2+),yTb~(3+) blue-emitting phosphors via crystal field modulation and energy transfer

摘要

Two series of (K_(1-x)Na_x)SrPO4:0.005Eu~(2+) and K_(0.4)Na_(0.6)Sr_(0.995-y)PO4:0.005Eu~(2+), yTb~(3+) 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-x)Na_x)SrPO4:0.005Eu~(2+) host, tuning the emission from 426 to 498 nm. The red-shifted emission is attributed to an increased crystal field splitting for Eu~(2+) 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-x)Na_xSr_(0.995)PO4:0.005Eu~(2+) phosphor is very homogenous. Additionally, utilizing the principle of energy transfer, the emission color can be further tuned by co-doping with Tb~(3+). The chromaticity coordinates for the co-doped phosphor, K_(0.4)Na_(0.6)Sr_(0.995-y)PO4:0.005Eu~(2+), yTb~(3+), can be adjusted from (0.202, 0.406) fory = 0 to (0.232, 0.420) for y = 0.09. The energy transfer processes from the sensitizer (Eu~(2+)) 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 A using a concentration quenching method.