Europium Luminescence as a Structural Probe: Structure-Dependent Changes in Eu3+-Substituted Th(C2O4)2•xH2O (x=6, 2, and 0)

Three different phases of thorium(IV) oxalate hydrate [Th(C2O4)(2)xH(2)O] with different water contents, namely, the hexahydrate, dihydrate, and anhydrous phases, were prepared by thermal treatment of the as-precipitated hydrated thorium(IV) oxalates. With the change in water content, the structure changes from monoclinic (C2/m) for the hexahydrate to monoclinic (C2/c) for the dihydrate and finally to a highly disordered phase for the anhydrous state. The local coordination around the Th4+ ions in these structures was monitored by using Eu3+ ions as luminescent probes. The analyses of the emission spectra owing to the electric-dipole transitions (EDTs) and magnetic-dipole transitions (MDTs) of the Eu3+ ions in the different phases revealed that the site symmetry around the Th4+ ions decreases systematically from symmetric to highly distorted with the decrease in water content. The relative intensity ratios (I) of the EDT to MDT for x = 6, 2, and 0 are 2.36, 3.32, and 4.23, respectively. The analyses of the delay curves corresponding to the D-5(0) level of the Eu3+ ions indicated biexponential decay with varying lifetimes, which suggests two possible environments around the Eu3+ ions and a possible role of oxygen vacancies. The long-lived species is expected to be distant from the charge-compensation defects, whereas the short-lived species is nearer to the charge-compensation defect. Tunability of the emission with the excitation wavelength is observed for all phases and is attributed to the presence of defect-mediated broad emission in the blue region and red-orange emission from the Eu3+ ions.