Eu2+/Eu3+-emission-ratio-tunable CaZr(PO4)2:Eu phosphors synthesized in air atmosphere for potential white light-emitting deep UV LEDs

Abstract

Mixed-valence Eu-doped CaZr(PO₄)₂ was prepared in air atmosphere by a two-step solid-state reaction for the first time. The reduction of Eu³⁺ to Eu²⁺ in air atmosphere was observed in the second synthesis step via the use of NH₄H₂PO₄ and the first-step product CaZrO₃:Eu³⁺ to synthesize CaZr(PO₄)₂:Eu, which strengthens the evidence linking the tetrahedral anion groups PO₄ with the Eu reduction. This reduction mechanism is explained by a charge compensation model. The luminescence measurements indicate that the obtained phosphors exhibit a broad bluish-green fluorescence of Eu²⁺ and a sharp orange-red emission of Eu³⁺ on 200–350 nm ultraviolet (UV) excitations. A tunable white luminescence covering the whole visible spectrum is realized based on the gradual reduction of Eu³⁺ to Eu²⁺ with increasing the sintering time. The optimal white luminescence excited at 247 nm presents the CIE color coordinates of (0.322, 0.305), a correlated color temperature of 6243 K, and a good thermal stability. Furthermore, energy transfer from Eu²⁺ to Eu³⁺ is found in CaZr(PO₄)₂:Eu. Our investigation of CaZr(PO₄)₂:Eu may provide a practical basis to design and fabricate novel phosphors for white light-emitting diodes (W-LEDs).