High-Temperature Synthesis of Ferromagnetic EuTa(O,N) with a Triple Perovskite Structure.

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N or N/H gas. The nitrogen stoichiometry was tuned from 0.63 to 1.78 atoms per Eu or Ta atom, starting with appropriate N/O ratios in the mixture of the reactants EuO, EuN and TaN, or EuO and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaON and EuTaON, showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaON is a simple perovskite with cubic 3̅ structure and cell parameter = 4.02043(1) Å, whereas the new compound EuTaON is the first example of a triple perovskite oxynitride and shows space group 4/ with crystal parameters = 3.99610(2), = 11.96238(9) Å. The tripling of the c-axis in this phase is a consequence of the partial ordering of europium atoms with different charges in two A sites of the perovskite structure with relative ratio 2:1, where the formal oxidation states +3 and +2 are respectively dominant. Magnetic data provide evidence of ferromagnetic ordering developing at low temperatures in both oxynitrides, with saturation magnetization of about 6 μ and 3 μ per Eu ion for EuTaON and the triple perovskite EuTaON respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu in the latter compound.