Controlling the activator site to tune europium valence in oxyfluoride phosphors

A new Eu ³⁺-activated oxyfluoride phosphor Ca ₁₂Al ₁₄O ₃₂F ₂:Eu ³⁺ (CAOF:Eu ³⁺) was synthesized by a solid state reaction. Commonly red line emission was detected in the range of 570-700 nm. To achieve the requirement of illumination, this study revealed a crystal chemistry approach to reduce Eu ions from 3+ to 2+ in the lattice. Replacing Al ³⁺-F ^- by the appreciate dopant Si ⁴⁺-O ^2- is adopted to enlarge the activator site that enables Eu ³⁺ to be reduced. The crystallization of samples was examined by powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Photoluminescence results indicated that as-synthesized phosphors Ca ₁₂Al _14-zSi _zO _32+zF _2-z:Eu (z = 0-0.5, CASOF:Eu) display an intense blue emission peaking at 440 nm that was produced by 4f-5d transition of Eu ²⁺, along with the intrinsic emission of Eu ³⁺ under UV excitation. Moreover, the effect of Si ⁴⁺-O ^2- substitution involved in the coordination environment of the activator site was investigated by further crystallographic data from Rietveld refinements. The ¹⁹F solid-state nuclear magnetic resonance (NMR) data were in agreement with refinement and photoluminescence results. Furthermore, the valence states of Eu in the samples were analyzed with the X-ray absorption near edge structure (XANES). The quantity of substituted Si ⁴⁺-O ^2- tunes chromaticity coordinates of Ca ₁₂Al _14-zSi _zO _32+zF _2-z:Eu phosphors from (0.6101, 0.3513) for z = 0 to (0.1629, 0.0649) for z = 0.5, suggesting the potential for developing phosphors for white light emitting diodes (WLEDs). Using an activator that is valence tunable by controlling the size of the activator site represents a hitherto unreported structural motif for designing phosphors in phosphor converted light emitting diodes (pc-LEDs).