Controlled synthesis of layered rare-earth hydroxide nanosheets leading to highly transparent (Y_0.95Eu_0.05)₂O₃ ceramics

Chemical precipitation at the freezing temperature of ∼4°C has directly yielded layered rare-earth hydroxide [LRH, Ln₂(OH)₅NO₃·nH₂O, Ln = Y_0.95Eu_0.05] 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.