Layered rare-earth hydroxides (LRHs) of (Y1-xEu x)2(OH)5NO3· n H2O (x = 0-1): Structural variations by Eu3+ doping, phase conversion to oxides, and the correlation of photoluminescence behaviors

Abstract

Layered hydroxides of (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>(OH) <inf>5</inf>NO<inf>3</inf>nH<inf>2</inf>O (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 C<inf>4v</inf> to C<inf>1</inf> symmetries and that the C<inf>1</inf>-site Eu³⁺ is significantly associated with the 595 nm ⁵D<inf>0</inf>→⁷F <inf>1</inf> and the 615 nm ⁵D<inf>0</inf>→⁷F <inf>2</inf> transitions while the C<inf>4v</inf>-site Eu³⁺ with the 589 nm ⁵D<inf>0</inf>→⁷F<inf>1</inf> and the 698 nm ⁵D<inf>0</inf>→⁷F<inf>4</inf> transitions. The hydroxides convert to cubic (Y<inf>1-x</inf>Eu<inf>x</inf>)<inf>2</inf>O <inf>3</inf> 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(⁵D<inf>0</inf>→⁷F <inf>2</inf>)/I(⁵D<inf>0</inf>→⁷F<inf>1</inf>), 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 C<inf>1</inf> symmetry from distorted S<inf>6</inf> sites. © 2010 American Chemical Society.