Sr2Mn3Sb2O2 Type Oxyselenides: Structures, Magnetism, and Electronic Properties of Sr2AO2M2Se2 (A=Co, Mn; M=Cu, Ag)

Four new oxyselenides with nominal formula Sr(2)AO(2)M(2)Se(2) (A=Co, Mn; M=Cu, Ag) have been synthesized. They all crystallize in an I4/mmm space group and consist of alternating perovskite-like (Sr(2)AO(2))(2+) blocks and antiflourie (M2Se2)(2-) layers, which are relatively rare layered oxyselenides reported so far that are isostructural to Sr2Mn3Sb2O2. From powder X-ray diffraction data, compounds Sr2CoO2Cu2Se2 and Sr2CoO2Ag2Se2 are found near stoichiometric, whereas Sr2MnO2Cu2-delta Se2 and Sr2MnO2Ag2-delta Se2 possess substantial copper or silver vacancies (delta approximate to 0.5), consistent with their oxysulfide analogues. X-ray photoelectron spectroscopy measurements indicate the readily oxidization of Mn2+ ions should be responsible for the occurrence of Cu/Ag vacancies. The rigid (Sr(2)AO(2))(2+) blocks within these compounds constrain the basal lattice parameters in the ab plane and result in largely deformed tetrahedral sites for the large silver ions. Magnetic susceptibility measurements of Sr2CoO2M2Se2 (M=Cu, Ag) show complex antiferromagnetic transitions, while Sr2MnO2M2-delta Se2 (M=Cu, Ag) show high-temperature Curie-Weiss behavior, followed by low-temperature antiferromagnetic transitions at 54 K and 67 K, respectively. Except for Sr2MnO2Ag2-delta Se2, the other three compounds exhibit p-type semiconducting transport properties, with the measured resistivities several orders lower than their oxysulfide analogues. Hall measurement reveals high mobilities of Sr2CoO2M2Se2 (M=Cu, Ag) compounds at room temperature. The unusually small optical band gaps (similar to 0.07 eV) of Sr2CoO2Cu2Se2, Sr2CoO2Ag2Se2, and Sr2MnO2Cu2-delta Se2 are also reported.