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

Abstract

Emission spectral tuning is an important issue for phosphors. Herein, we report a Eu²⁺-activated solid-solution LiSr_1−xBa_xPO₄ 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 LiSr_0.995PO₄:0.005Eu²⁺ (x = 0) to LiSr_0.095Ba_0.9PO₄:0.005Eu²⁺ (x = 0.9). LiBa_0.995PO₄:0.005Eu²⁺ 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 LiSr_0.695Ba_0.3PO₄:0.005Eu²⁺. The emission intensity of the phosphors at 125 °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.