Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe${}^{4+}$-containing perovskite-related phase Sr${}_3$Fe${}_2$O${}_{7-\delta }$ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound ($\delta =0$) exhibits a charge-disproportionation transition at $T_D=340$ K, and an antiferromagnetic transition at $T_N=115$ K. For temperatures $T\le T_D$, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe${}^{4+}$ moments. The fully deoxygenated compound ($\delta =1$) is highly insulating, and its Fe${}^{3+}$ moments exhibit commensurate antiferromagnetic order below $T_N\sim 600$ K. Compounds with intermediate $\delta$ exhibit different order with lower $T_N$, likely as a consequence of frustrated exchange interactions between Fe${}^{3+}$ and Fe${}^{4+}$ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.

• Received 24 January 2013

DOI:https://doi.org/10.1103/PhysRevB.87.214410