Spin Frustration in Double Perovskite Oxides and Oxynitrides: Enhanced Frustration in La 2 MnTaO 5 N with a Large Octahedral Rotation.

The B-site sublattice in the double perovskite oxides A ₂ BB'O ₆ (B: magnetic cation; B': nonmagnetic cation) causes spin frustration, but the relationship between the structure and spin frustration remains unclear although a number of compounds have been studied. The present study systematically investigated A ₂ Mn ^II B'O ₆ ( S = 5/2) and found that the frustration factor, defined by f = |θ _W |/ T _N (θ _W : Weiss temperature; T _N : Néel temperature), scales linearly with the tolerance factor t , i.e., octahedral rotation. Unexpectedly, La ₂ MnTaO ₅ N (space group: P 2 ₁ / n ) synthesized under high pressure is more frustrated ( f = 6) than oxides with similar t values, despite the large octahedral rotation due to the small t value of 0.914. Structural analysis suggests that the enhanced frustration can be attributed to the site preference of nitride anions at the equatorial positions, which reduces the variance of neighboring Mn-Mn distances. Our findings provide a new guide to control and improve spin frustration in double perovskites with multiple anions.