Nitride Tuning of Magnetic Frustration in the Double\nPerovskite Ba<sub>2</sub>MnWO<sub>6</sub>

The\nnew double perovskite oxynitride Ba<sub>2</sub>MnWO<sub>4.42</sub>N<sub>1.58</sub> has been obtained by topochemical ammonolysis at\n700 °C of B-site ordered Ba<sub>2</sub>MnWO<sub>6</sub> using\na high NH<sub>3</sub> flow rate of 600 cm<sup>3</sup>/min. The relatively\nlow synthesis temperature hinders the cation mobility, allowing the\norder of Mn and W cations of the precursor oxide to be unperturbed\nin the oxynitride. Synchrotron X-ray diffraction, electron diffraction,\nand neutron diffraction indicate that Ba<sub>2</sub>MnWO<sub>4.42</sub>N<sub>1.58</sub> crystallizes in the <i>Fm</i>3̅<i>m</i> space group with a larger parameter of 8.2434(5) Å\ncompared to Ba<sub>2</sub>MnWO<sub>6</sub> [8.20337(1) Å]. Magnetic\nsusceptibility measurements show that Ba<sub>2</sub>MnWO<sub>4.42</sub>N<sub>1.58</sub> orders antiferromagnetically below <i>T</i><sub>N</sub> = 3.8 K, and the observed Curie–Weiss temperature\nθ<sub>CW</sub> = −70(3) K indicates a frustrated Mn lattice\nwith frustration parameter <i>f</i> = |θ<sub>CW</sub>|/<i>T</i><sub>N</sub> ≈ 18, which is significantly\nlarger than for the oxide (<i>f</i> ≈ 7.2). The substitution\nof oxide by the nitride anion induces the oxidation of Mn<sup>2+</sup> to Mn<sup>3+/4+</sup> and a subsequent decrease of the paramagnetic\neffective moment from 6.28 to 5.16 μ<sub>B</sub>/f.u. The observation\nin the oxynitride of a frustration parameter which is larger than\ntwice that of the oxide precursor is rationalized as caused by the\nrelative enhancement of the nearest neighbors’ magnetic interactions\n(<i>J</i><sub>1</sub>) Mn–(O/N)–(O/N)–Mn\nat 90°, compared to next-nearest neighbors’ interactions\nat 180° (<i>J</i><sub>2</sub>) Mn–(O/N)–W–(O/N)–Mn,\ndue to the smaller electronegativity of nitrogen compared to oxygen\nthat increases the covalency of bonding. These results expand the\nchemical and structural diversity of complex transition metal oxynitrides\nand provide a new route to tailor spin frustration in transition metal\ndouble perovskites.