Ammonothermal Synthesis of the Mixed‐Valence Nitrogen‐Rich Europium Tantalum Ruddlesden‐Popper Phase EuIIEuIII2Ta2N4O3

The mixed‐valence europium tantalum nitride oxide EuIIEuIII2Ta2N4O3 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. EuIIEuIII2Ta2N4O3 was obtained as a black microcrystalline powder. Single‐crystal synchrotron diffraction data revealed a Ruddlesden‐Popper phase crystallizing in space group P42/mnm (no. 136) with a = 5.7278(1), c = 19.8149(5) Å 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). EuII and EuIII 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. The first nitrogen‐rich Ruddlesden‐Popper phase with mixed‐valence europium, Eu3Ta2N4O3, was synthesized ammonothermally using custom‐built high‐pressure autoclaves. X‐ray diffraction with microfocused synchrotron radiation and EDX analyses confirm crystal‐chemical calculations with respect to Eu(II)/Eu(III) and N/O ordering. Diffuse reflectance spectra reveal a band gap of 0.6 eV.