Controlling The Activator Site To Tune Europium Valence in Oxyfluoride Phosphors

Abstract

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).

Crystal-Site Engineering Control for the Reduction of Eu3+ to Eu2+ in CaYAlO4: Structure Refinement and Tunable Emission Properties

“…49 Thus, it can be concluded that the local environment and size of the replaced crystal-site (Ca 2+ in this system) play a dominant role in the reduction of Eu 3+ . 12,50 Through the crystal-site engineering approach, we introduced Si 4+ −Ca 2+ into the CaYAlO 4 host to replace Al 3+ −Y 3+ attempting to shrink the AlO 6 octahedrons, accompanied by the expansion of CaO 9 polyhedron, then fulfilling the conditions for the reduction of Eu 3+ , because Si 4+ has a smaller radius than Al 3+ and substitution of Y 3+ by Ca 2+ can achieve charge compensation in the whole structure. 11,12,51 As the XRD pattern shows in Supporting Information Figure S3, impurities began to appear when x exceeded 0.30; thus, we restricted the x value to a maximum of 0.30.…”

“…12,50 Through the crystal-site engineering approach, we introduced Si 4+ −Ca 2+ into the CaYAlO 4 host to replace Al 3+ −Y 3+ attempting to shrink the AlO 6 octahedrons, accompanied by the expansion of CaO 9 polyhedron, then fulfilling the conditions for the reduction of Eu 3+ , because Si 4+ has a smaller radius than Al 3+ and substitution of Y 3+ by Ca 2+ can achieve charge compensation in the whole structure. 11,12,51 As the XRD pattern shows in Supporting Information Figure S3, impurities began to appear when x exceeded 0.30; thus, we restricted the x value to a maximum of 0.30. First, Rietveld refinement with GSAS program has been performed to reveal more details to clarify how substitution (Si 4+ −Ca 2+ for Al 3+ −Y 3+ ) affects the Ca 0.99+x Y 1−x Al 1−x Si x O 4 :Eu 0.01 (CYASO, x = 0−0.30) crystal structure and luminescence properties.…”

“…The corresponding fine structure of CYASO:Eu (x = 0−0.30) is further examined by HRTEM as displayed in the inset of Supporting Information Figure S4, which presents a very uniform contrast without significant defects, indicating the high crystallinity. Table 2 12,49 Therefore, the unit cell contraction caused by the substitution of Al 3+ −Y 3+ by Si 4+ −Ca 2+ is presented in Figure 3c. However, the simultaneous contraction and expansion of the lattice parameters in different directions causes increased tilting of the AlO 6 octahedrons and greater distortion of the AlO 6 octahedrons and (Ca/Y)O 9 polyhedrons, which would greatly affect the local environment of activators and the corresponding luminescence properties as discussed in the following paragraphs.…”

“…31−34 However, less research has been conducted on the reduction of Eu 3+ through crystal-site engineering approach by modifying the coordination environment and crystal site size of activators, to then fulfill the coexistence of luminescence from Eu 3+ and Eu 2+ in single phased phosphors. 11,12,35,36 Accordingly, it is a promising method to overcome the limitations of Eu 3+ activated phosphors via valence transfer, combined with the luminescence of Eu 2+ to obtain warm white light.…”

Dopant distribution and influence of sonication temperature on the pure red light emission of mixed oxide phosphor for solid state lighting

“…Now in this case v opt (O) = 3.2 and v uncorr (Eu) = 1.78 which implies the energy of the O 2p ? Eu 4f CT band as 42,600 cm À1 and it should be near 235 nm [25]. The presence of the CT band indicates the strong covalency between Eu 3+ and O 2À in the prepared phosphors [26].…”