High-Temperature Synthesis of Ferromagnetic Eu 3 Ta 3 (O,N) 9 with a Triple Perovskite Structure.

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N 2 or N 2 /H 2 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 Eu 2 O 3 , EuN and Ta 3 N 5 , or Eu 2 O 3 and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO 2.37 N 0.63 and Eu 3 Ta 3 O 3.66 N 5.34 , showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO 2.37 N 0.63 is a simple perovskite with cubic Pm 3̅ m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu 3 Ta 3 O 3.66 N 5.34 is the first example of a triple perovskite oxynitride and shows space group P 4/ 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 2.37 N 0.63 and the triple perovskite Eu 3 Ta 3 O 3.66 N 5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu 2+ in the latter compound.