Structures and physical properties of layered oxyselenides Ba(2)MO(2)Ag(2)Se(2) (M = Co, Mn)

Two new layered oxyselenides, Ba2MO2Ag2Se2 (M = Co, Mn), have been successfully synthesized via solid-state reaction. It is found that these two compounds, consisting of the infinite MO2 square planes and antifluorite-type Ag2Se2 layers separated by barium, possess new structural features while keeping I4/mmm symmetry. A detailed calculation on the discrete coordination of transition metals by oxygen in the two compounds and Ba2ZnO2Ag2Se2 revealed quite different energy landscapes. The calculated results indicate that the manganese compound favors adoption of the I4/mmm space group, while the cobalt compound could be at the boundary of the transition between the I4/mmm and Cmca phases. In Ba2CoO2Ag2Se2, the coexistence of a large barium ion and a Ag2Se2 layer expands the oxide layer significantly and results in the largest Co-O bond length in the square-planar sheet ever reported. Ba2CoO2Ag2Se2 is near-stoichiometric, whereas Ba2MnO2Ag2Se2 contains 7% silver vacancies, which is explained by the mixed valence of the manganese ion between 2+ and 3+. In Ba2CoO2Ag2Se2, the zero-field-cooled and field-cooled susceptibilities bifurcate at 159 K, located between two antiferromagnetic (AFM) transitions. Meanwhile, Ba2MnO2Ag2Se2 shows high-temperature Curie-Weiss behavior, followed by a low-temperature AFM transition with TN = 32 K. They both exhibit semiconducting behavior with resisitivities of about 10(5)Ω cm at room temperature. The optical band gaps are determined to be 1.49 and 1.18 eV for Ba2CoO2Ag2Se2 and Ba2MnO2Ag2Se2, respectively. Band structure calculations reveal that Ba2CoO2Ag2Se2 is a direct-gap semiconductor, with a calculated band gap of 1.147 eV; however, Ba2MnO2Ag2Se2 failed to reproduce the semiconducting behavior within an A-type AFM model.