Sr 2 MnO 2 Na 1.6 Se 2 : A Metamagnetic Layered Oxychalcogenide Synthesized by Reductive Na Intercalation to Break [Se 2 ] 2- Perselenide Dimer Units.

Recent advances in anion-redox topochemistry have enabled the synthesis of metastable mixed-anion solids. Synthesis of the new transition metal oxychalcogenide Sr 2 MnO 2 Na 1.6 Se 2 by topochemical Na intercalation into Sr 2 MnO 2 Se 2 is reported here. Na intercalation is enabled by the redox activity of [Se 2 ] 2- perselenide dimers, where the Se-Se bonds are cleaved and a [Na 2- x Se 2 ] (2+ x )- antifluorite layer is formed. Freshly prepared samples have 16(1) % Na-site vacancies corresponding to a formal oxidation state of Mn of +2.32, a mixed-valence between Mn 2+ (d 5 ) and Mn 3+ (d 4 ). Samples are highly prone to deintercalation of Na, and over two years, even in an argon glovebox environment, the Na content decreased by 4(1) %, leading to slight oxidation of Mn and a significantly increased long-range ordered moment on the Mn site as measured using neutron powder diffraction. The magnetic structure derived from neutron powder diffraction at 5 K reveals that the compound orders magnetically with ferromagnetic MnO 2 sheets coupled antiferromagnetically. The aged sample shows a metamagnetic transition from bulk antiferromagnetic to ferromagnetic behavior in an applied magnetic field of 2 T, in contrast to the Cu analogue, Sr 2 MnO 2 Cu 1.55 Se 2 , where there is only a hint that such a transition may occur at fields exceeding 7 T. This is presumably due to the higher ionic character of [Na 2- x Se 2 ] (2+ x )- layers compared to [Cu 2- x Se 2 ] (2+ x )- layers, reducing the strength of the antiferromagnetic interactions between MnO 2 sheets. Electrochemical Na intercalation into Sr 2 MnO 2 Se 2 leads to the formation of multiphase sodiated products. The work shows the potential of anion redox to yield novel compounds with intriguing physical properties.