Nitride Tuning of Magnetic Frustration in the Double Perovskite Ba₂MnWO₆

The new double perovskite oxynitride Ba₂MnWO_4.42N_1.58 has been obtained by topochemical ammonolysis at 700 °C of B-site ordered Ba₂MnWO₆ using a high NH₃ flow rate of 600 cm³/min. The relatively low synthesis temperature hinders the cation mobility, allowing the order of Mn and W cations of the precursor oxide to be unperturbed in the oxynitride. Synchrotron X-ray diffraction, electron diffraction, and neutron diffraction indicate that Ba₂MnWO_4.42N_1.58 crystallizes in the Fm3̅m space group with a larger parameter of 8.2434(5) Å compared to Ba₂MnWO₆ [8.20337(1) Å]. Magnetic susceptibility measurements show that Ba₂MnWO_4.42N_1.58 orders antiferromagnetically below T _N = 3.8 K, and the observed Curie-Weiss temperature θ_CW = -70(3) K indicates a frustrated Mn lattice with frustration parameter f = |θ_CW|/T _N ≈ 18, which is significantly larger than for the oxide (f ≈ 7.2). The substitution of oxide by the nitride anion induces the oxidation of Mn²⁺ to Mn^3+/4+ and a subsequent decrease of the paramagnetic effective moment from 6.28 to 5.16 μ_B/f.u. The observation in the oxynitride of a frustration parameter which is larger than twice that of the oxide precursor is rationalized as caused by the relative enhancement of the nearest neighbors' magnetic interactions (J ₁) Mn-(O/N)-(O/N)-Mn at 90°, compared to next-nearest neighbors' interactions at 180° (J ₂) Mn-(O/N)-W-(O/N)-Mn, due to the smaller electronegativity of nitrogen compared to oxygen that increases the covalency of bonding. These results expand the chemical and structural diversity of complex transition metal oxynitrides and provide a new route to tailor spin frustration in transition metal double perovskites.