The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6 GPa and 1400°C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O2- and N3- anions. MnTaO2N with an "orbital-inactive" Mn2+ ion (d5; S = 5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ ≈ 0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O2-/N3- species that results in a distribution of (super)-super-exchange interactions.
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