Sr₂MnO₂Na_1.6Se₂: A Metamagnetic Layered Oxychalcogenide Synthesized by Reductive Na Intercalation to Break [Se₂]^2-...

Recent advances in anion-redox topochemistry have enabled the synthesis of metastable mixed-anion solids. Synthesis of the new transition metal oxychalcogenide Sr₂MnO₂Na_1.6Se₂ by topochemical Na intercalation into Sr₂MnO₂Se₂ is reported here. Na intercalation is enabled by the redox activity of [Se₂]^2- perselenide dimers, where the Se-Se bonds are cleaved and a [Na_2- Se₂]⁽²⁺ 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 Mn²⁺ (d⁵) and Mn³⁺ (d⁴). 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 MnO₂ 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, Sr₂MnO₂Cu_1.55Se₂, 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- Se₂]⁽²⁺ layers compared to [Cu_2- Se₂]⁽²⁺ layers, reducing the strength of the antiferromagnetic interactions between MnO₂ sheets. Electrochemical Na intercalation into Sr₂MnO₂Se₂ leads to the formation of multiphase sodiated products. The work shows the potential of anion redox to yield novel compounds with intriguing physical properties.