Eu₃Ta₂ON₅: an n = 2 Ruddlesden-Popper ferromagnetic oxynitride with long-range ordered anion vacancies

The new compound Eu₃Ta₂ON₅ with an n = 2 Ruddlesden-Popper structure has been prepared by solid state reaction between EuN, Eu₂O₃ and Ta₃N₅ at 1200 °C under N₂. It crystallizes in the I4/mmm space group with unit cell parameters a = 3.98240(1) Å and c = 20.42020(6) Å and has anion vacancies in central A-site layers of the perovskite blocks, where tantalum is in a square pyramidal coordination polyhedron with composition [TaO_0.5N_4.5] and europium is 8-coordinated. The perovskite blocks intergrow with [EuO_0.5N_0.5] NaCl-type layers of the Ruddlesden-Popper structure. Electron Energy Loss spectrum imaging shows a mixed Eu²⁺/Eu³⁺ valence with preferred occupation of Eu³⁺ in the 8-coordinated sites. Magnetic susceptibility measurements are consistent with the formal composition Eu₂²⁺Eu³⁺Ta⁵⁺₂ON₅ and indicate ferromagnetic order of Eu²⁺ ions below T_C = 20 K. Interestingly, despite the oxynitride containing non-magnetic Eu³⁺ ions (⁷F₀), the Curie temperature is found to be much larger than in related Eu₂TiO₄ and Eu₃Ti₂O₇ Ruddlesden-Popper compounds (≈9-10 K), which is attributed to the covalency-reinforced Eu²⁺-N/O-Eu²⁺ superexchange interactions.