Tuning crystal structure and luminescence of Eu2+-activated LiSr1-xBaxPO4 solid solution for white light-emitting diodes

Emission spectral tuning is an important issue for phosphors. Herein, we report a Eu2+-activated solid-solution LiSr1-xBaxPO4 based on a recently discovered monoclinic LiSrPO4, which has only one Sr2+ crystallographic site. Structural refinement indicates that the crystal structure can be maintained in the monoclinic phase from LiSr0.995PO4:0.005Eu(2+) (x = 0) to LiSr0.095Ba0.9PO4:0.005Eu(2+) (x = 0.9). LiBa0.995PO4:0.005Eu(2+) crystallizes in a trigonal phase, with different orientations of LiO4 and PO4 tetrahedrons from the monoclinic one. Both Sr2+ and Ba2+ are situated in the P6Li6 cage that is composed of six PO4 and six LiO4 tetrahedrons. However, different orientations of LiO4 and PO4 tetrahedrons results in different coordination situations for Sr2+ and Ba2+, which leads to two different emission bands at 420 and 470 nm for Eu2+ activation. It is interesting that the change of Sr2+/Ba2+ polyhedral size through solid solution does not result in a continuous shift of emission band from 420 to 470 nm, but the variation of intensity of two bands individually, which leads to the observation of both emission bands on LiSr0.695Ba0.3PO4:0.005Eu(2+). The emission intensity of the phosphors at 125 degrees C remains 75% of the value at room temperature, suggesting good thermal stability. The performance of phosphor-converted light-emitting diodes (pc-LEDs) indicates that phosphors can be used for near-UV chip-based white pc-LEDs.