Controlling The Activator Site To Tune Europium Valence in Oxyfluoride Phosphors

• Published in: Chemistry of materials 2012-06, Vol.24 (11), p.2220-2227
• Main Authors: Huang, Kuan-Wei, Chen, Wei-Ting, Chu, Cheng-I, Hu, Shu-Fen, Sheu, Hwo-Shuenn, Cheng, Bing-Ming, Chen, Jin-Ming, Liu, Ru-Shi
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm3011327

A new Eu3+-activated oxyfluoride phosphor Ca12Al14O32F2:Eu3+ (CAOF:Eu3+) 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 Al3+–F– by the appreciate dopant Si4+–O2– is adopted to enlarge the activator site that enables Eu3+ 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 Ca12Al14‑z Si z O32+z F2–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 Eu2+, along with the intrinsic emission of Eu3+ under UV excitation. Moreover, the effect of Si4+–O2– 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 Si4+–O2– tunes chromaticity coordinates of Ca12Al14–z Si z O32+z F2–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).