Structures and Physical Properties of Layered Oxyselenides Ba2MO2Ag2Se2 (M = Co, Mn)

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• 作者：Tingting Zhou ; Yumei Wang ; Shifeng Jin ; Dandan Li ; Xiaofang Lai ; Tianping Ying ; Han Zhang ; Shijie Shen ; Wenjun Wang ; Xiaolong Chen
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：April 21, 2014
• 年：2014
• 卷：53
• 期：8
• 页码：4154-4160
• 全文大小：462K
• 年卷期：v.53,no.8(April 21, 2014)
• ISSN：1520-510X

文摘

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鈥揙 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鈥揥eiss behavior, followed by a low-temperature AFM transition with T_N = 32 K. They both exhibit semiconducting behavior with resisitivities of about 10⁵惟 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.