Sr₂Mn₃Sb₂O₂ Type Oxyselenides: Structures, Magnetism,\nand Electronic Properties\nof Sr₂<i>A</i>O₂<i>M</i>₂Se₂ (<i>A</i>=Co, Mn; <i>M</i>=Cu, Ag)

Abstract

Four new oxyselenides with nominal formula Sr₂<i>A</i>O₂<i>M</i>₂Se₂ (<i>A</i>=Co, Mn; <i>M</i>=Cu, Ag) have\nbeen\nsynthesized. They all crystallize in an <i>I</i>4/<i>mmm</i> space group and consist of alternating perovskite-like\n(Sr₂<i>A</i>O₂)²⁺ blocks\nand antiflourie (<i>M</i>₂Se₂)^2‑ layers, which are relatively rare layered oxyselenides\nreported so far that are isostructural to Sr₂Mn₃Sb₂O₂. From powder X-ray diffraction data,\ncompounds Sr₂CoO₂Cu₂Se₂ and Sr₂CoO₂Ag₂Se₂ are\nfound near stoichiometric, whereas Sr₂MnO₂Cu_2‑δSe₂ and Sr₂MnO₂Ag_2‑δSe₂ possess substantial copper\nor silver vacancies (δ≈0.5), consistent with their oxysulfide\nanalogues. X-ray photoelectron spectroscopy measurements indicate\nthe readily oxidization of Mn²⁺ ions should be responsible\nfor the occurrence of Cu/Ag vacancies. The rigid (Sr₂<i>A</i>O₂)²⁺ blocks within these compounds\nconstrain the basal lattice parameters in the <i>ab</i> plane\nand result in largely deformed tetrahedral sites for the large silver\nions. Magnetic susceptibility measurements of Sr₂CoO₂<i>M</i>₂Se₂ (<i>M</i>=Cu, Ag) show complex antiferromagnetic transitions, while Sr₂MnO₂<i>M</i>_2‑δSe₂ (<i>M</i>=Cu, Ag) show high-temperature\nCurie–Weiss behavior, followed by low-temperature antiferromagnetic\ntransitions at 54 K and 67 K, respectively. Except for Sr₂MnO₂Ag_2‑δSe₂, the other\nthree compounds exhibit p-type semiconducting transport properties,\nwith the measured resistivities several orders lower than their oxysulfide\nanalogues. Hall measurement reveals high mobilities of Sr₂CoO₂<i>M</i>₂Se₂ (<i>M</i>=Cu, Ag) compounds at room temperature. The unusually small\noptical band gaps (∼0.07 eV) of Sr₂CoO₂Cu₂Se₂, Sr₂CoO₂Ag₂Se₂, and Sr₂MnO₂Cu_2‑δSe₂ are also reported.