Cation exchange route to a Eu(II)-containing tantalum oxide

Traditional synthetic efforts to prepare Eu(II)-containing oxides have principally involved the use of high temperature reactions starting from EuO or a controlled, highly-reducing, atmosphere. Conversely, chimie douce approaches that are more amenable to the targeted syntheses of new, and potentially metastable, Eu(II)-oxides have yet to be explored. Herein, a cation-exchange route to new Eu(II)-containing oxides, e.g., EuTa4-xO11 (x = 0.04), has been discovered and its structure determined by powder X-ray diffraction (Space group P6322 (#182), a = 6.2539(2) angstrom; c = 12.3417(2) angstrom). The compound derives from the cation exchange of Na2Ta4O11, via a reaction with EuBr2 at 1173 K, and replacement by half the number of divalent Eu cations. Rietveld refinements show preferential ordering of the Eu cations over one of the two possible cation sites, i.e., Wyckoff site 2d (-94%; Eu1) versus 2b (-6%; Eu2). Total energy calculations confirm an energetic preference of the Eu cation in the 2d site. Tantalum vacancies of-1% occur within the layer of Eu cations and TaO6 octahedra, and-20% partial oxidation of Eu(II) to Eu(III) cations from charge balance considerations. 151Eu Mo center dot ssbauer spectroscopy measured at 78 K found a Eu(II):Eu(III) ratio of 69:31, with a relatively broad line width of the former signal of Gamma = 7.6(2) mm s-1. Also, the temperature-dependent magnetic susceptibility could be fitted to a Curie Weiss expression, giving a lieff = 6.2 liB and theta CW =-10 K and confirming a mixture of Eu(II)/Eu(III) cations. The optical bandgap of EuTa4-xO11 was found to be-1.5 eV (indirect), significantly redshifted as compared to-4.1 eV for Na2Ta4O11. Spin-polarized electronic structure calculations show that this redshift stems from the addition of Eu 4f7 states as a higher -energy valence band. Thus, these results demonstrate a new cation-exchange approach that represents a useful synthetic pathway to new Eu(II)-containing ox-ides for tunable magnetic and optical properties.

Automated summary

A new cation-exchange approach has been discovered to synthesize Eu(II)-containing oxides. EuTa4-xO11 has been successfully prepared and characterized. The compound exhibits a mixture of Eu(II) and Eu(III) cations, and shows tunable magnetic and optical properties.