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Cordes, Niklas; Nentwig, Markus; Eisenburger, Lucien; Oeckler, Oliver; Schnick, Wolfgang (2019): Ammonothermal Synthesis of the Mixed-Valence Nitrogen-Rich Europium Tantalum Ruddlesden-Popper Phase (EuEu2Ta2N4O3)-Eu-II-Ta-III. In: European Journal of Inorganic Chemistry, No. 17: pp. 2304-2311
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The mixed-valence europium tantalum nitride oxide (EuEu2Ta2N4O3)-Eu-II-Ta-III was synthesized with the ammonothermal approach in high-pressure custom-built autoclaves. The reaction was performed at 1070 K and a maximum pressure of 170 MPa in an ammonobasic environment with NaN3 and NaOH as mineralizers. (EuEu2Ta2N4O3)-Eu-II-Ta-III was obtained as a black microcrystalline powder. Single-crystal synchrotron diffraction data revealed a Ruddlesden-Popper phase crystallizing in space group P4(2)/mnm (no. 136) with a = 5.7278(1), c = 19.8149(5) angstrom and Z = 4. The crystallographic results from single-crystal diffraction data have been confirmed by powder diffraction and TEM measurements. Anion positions were assigned to O and N based on bond-valence (BVS), lattice energy (MAPLE) and charge distribution calculations (CHARDI). Eu-II and Eu-III are crystallographically ordered. The band gap was estimated from UV/Vis measurements employing the Kubelka-Munk function to be 0.6 eV, which supports the black color and the mixed valence of europium.