Controlled Synthesis of Layered Rare‐Earth Hydroxide Nanosheets Leading to Highly Transparent (Y0.95Eu0.05)2O3 Ceramics
Chemical precipitation at the freezing temperature of ~4°C has directly yielded layered rare‐earth hydroxide [LRH, Ln2(OH)5NO3·nH2O, Ln = Y0.95Eu0.05] nanosheets (up to 7 nm thick) for the Y/Eu binary system, with the interlayer NO3 − exchangeable with SO4 2−. Calcining the sulfate derivative at 1100°C for 4 h produces well‐dispersed and readily sinterable Ln2O3 red phosphor powders (~14.8 m2/g) that can be densified into highly transparent ceramics via optimized vacuum sintering at the relatively low temperature of 1700°C for 4 h (average grain size ~14 μm; in‐line transmittance ~80% at the 613 nm Eu3+ emission or ~99% of the theoretical transmittance of Y2O3 single crystal). Our systematic studies also found that (1) the extent of SO4 2− exchange and the interlayer distance of LRH are both affected by the SO4 2−/Ln3+ molar ratio (R), and an almost complete exchange is achievable at R = 0.25 as expected from the chemical formula (one SO4 2− replaces two NO3 − for charge balance). The optimal R value for sintering, however, was found to be 0.03; (2) The Ln3+ concentration for LRH synthesis substantially affects properties of the resultant oxides, and hard agglomeration has been significantly reduced at the optimized Ln3+ concentration of 0.05–0.075 mol/L; (3) Sulfate exchange significantly alters the thermal decomposition pathway of LRH, and was found essential to produce well‐dispersed and highly sinterable oxide powders; (4) Both the oxide powders and transparent ceramics exhibit the typical red emission of Eu3+ at ~613 nm (the 5D0→7F2 transition) under charge‐transfer (CT) excitation. Red‐shifted CT band center, stronger excitation/emission, and shorter fluorescence lifetime were, however, observed for the transparent ceramics.
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Document Type: Research Article
Publication date: May 1, 2015