Layered Rare-Earth Hydroxides (LRHs) of (Y_1−<i>x</i>Eu_<i>x</i>)₂(OH)₅NO₃·<i>n</i>H₂O (<i>x</i> = 0−1): Structural Variations by Eu³⁺ Doping, Phase Conversion to Oxides, and the Correlation of Photoluminescence Behaviors

Abstract

Layered hydroxides of (Y1−xEux)2(OH)5NO3·nH2O (x = 0−1) have been hydrothermally synthesized under the optimized conditions of 120 °C and pH ∼7.0. Eu incorporation yields steadily smaller particles, elongation of the well-developed hexagon platelets, and linearly expanded ab planes of the layered structure. The interlayer distance (c/2), closely related to the hydration number n, is inversely proportional to the Eu content. The systematically changing photoluminescence behaviors allow to conclude that a lower hydration shifts the Eu3+ coordination from C4v to C1 symmetries and that the C1-site Eu3+ is significantly associated with the 595 nm 5D0→7F1 and the 615 nm 5D0→7F2 transitions while the C4v-site Eu3+ with the 589 nm 5D0→7F1 and the 698 nm 5D0→7F4 transitions. The hydroxides convert to cubic (Y1−xEux)2O3 at temperatures ≥400 °C while retaining the original morphologies. The best luminescence was observed for the oxides at x = 0.05 for the 613-nm red emission, and significant quenching of luminescence occurred at x > 0.10. Red shift of the charge-transfer excitation band was observed to be due to the elongated Eu−O bond arising from lattice expansion. The asymmetry factor of luminescence, I(5D0→7F2)/I(5D0→7F1), exhibits a sharp increase from ∼11.4 at x = 0.5 to ∼23 at x = 1.0, which has been ascribed to the splitting of C1 symmetry from distorted S6 sites.