Layered Rare-Earth Hydroxides (LRHs) of (Y1-xEux)2(OH)5NO3•nH2O (x=0-1): Structural Variations by Eu3+ Doping, Phase Conversion to Oxides, and the Correlation of Photoluminescence Behaviors

Layered hydroxides of (Y1-xEux)(2)(OH)(5)NO3 center dot nH(2)O (x = 0-1) have been hydrothermally synthesized under the optimized conditions of 120 degrees C and pH similar to 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-4v, to C-1 symmetries and that the C-1-site Eu3+ is significantly associated with the 595 nm D-5(0)-> F-7(1) and the 615 nm D-5(0)-> F-7(2) transitions while the C-4v-site Eu3+ with the 589 nm D-5(0)-> F-7(1) and the 698 nm D-5(0)-> F-7(4) transitions. The hydroxides convert to cubic (Y1-xEux)(2)O-3 at temperatures >= 400 degrees 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(D-5(0)-> F-7(2))/I(D-5(0)-> F-7(1)), exhibits a sharp increase from similar to 11.4 at x = 0.5 to 23 at x = 1.0, which has been ascribed to the splitting of C-1 symmetry from distorted S-6 sites.