High-Temperature Synthesis of Ferromagnetic Eu₃Ta₃(O,N)₉ with a Triple Perovskite Structure

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N₂ or N₂/H₂ 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 Eu₂O₃, EuN and Ta₃N₅, or Eu₂O₃ and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO_2.37N_0.63 and Eu₃Ta₃O_3.66N_5.34, showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO_2.37N_0.63 is a simple perovskite with cubic Pm3̅m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu₃Ta₃O_3.66N_5.34 is the first example of a triple perovskite oxynitride and shows space group P4/mmm with crystal parameters a = 3.99610(2), c = 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_B and 3 μ_B per Eu ion for EuTaO_2.37N_0.63 and the triple perovskite Eu₃Ta₃O_3.66N_5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu²⁺ in the latter compound.