The topochemical nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to the cubic, Fm-3m double perovskite oxynitride Sr2FeMoO4.9N1.1 where double-exchange interactions determine ferromagnetic order with Tc ≈ 100 K.  Substitution of oxide by nitride induces bond asymmetries and local electronically-driven structural distortions, which combined with Fermi level lowering restrict charge itineracy to confined regions and preclude spontaneous long-range magnetic order. Under a magnetic field, ferromagnetic correlations expand, favoring charge delocalization and a negative magnetoresistance is observed.

Topochemical nitridation of Sr2FeMoO6
Roberta Ceravola, Carlos Frontera, Judith Oró-Solé, Ashley P.Black, C.Ritter, Ignasi Mata, Elies Molins, Josep Fontcuberta and Amparo Fuertes
Chemical Communications 55, 3105-3108, 2019
DOI: 10.1039/C8CC09845J

Figure: (a) Schematic band filing of Sr2FeMoO6. Fermi level is nearby the bottom of Fe-3dt2g↓-Mo-(4d,5s)↓ bands. (b) In the oxide FeO6 and MoO6 octahedra are regular (top sketch) and the free carriers in the conduction band (bottom sketch) promote long range ferromagnetic order. (c) Schematic band filing of Sr2FeMoO4.9N1.1. (d) Localized states formed around defect-related potential wells (e.g.: nitride sites, Jahn-Teller Fe4+ ions). (e) Magnetoresistance (MR=[r(H)-r(0)]/r(0)) of the oxynitride sample at different temperatures; Inset: logarithm of the resistance (at zero field) as a function of T-1/4.

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