Mn₁₁Ta₄O₂₁ is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)₂ stacking sequence that is related to BaVO₃ and BaCrO₃ high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at T_N1 = 88 K and T_N2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below T_N1 and T_N2, respectively. A complementary description with 14H-(hhccccc)₂ sequence of only Mn octahedra provides a direct comparison with BaMnO_3−δ hexagonal perovskites and naturally explains the AFM order. Below T_N2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

中文翻译：

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Mn11Ta4O21的磁性结构及其作为六方A位锰矿的解释。

Mn ₁₁ Ta ₄ O ₂₁被表示为第一六角形的A位锰矿。根据简单的规则，该结构与14H层（cchchch）₂堆叠序列兼容，该序列与BaVO ₃和BaCrO ₃高压多晶型物有关。通过Ba和Mn之间离子半径大小的差异解释了A位化学计量过高。磁性能在T _N1 = 88 K和T _N2 = 56 K处显示出两个跃迁。中子粉末衍射表明，在T _N1和T以下具有纯反铁磁和亚铁磁级的两个磁结构_N2分别。与仅锰八面体的14H-（hhccccc）₂序列的补充描述提供了与BaMnO3 _-δ六方钙钛矿的直接比较，自然地解释了AFM顺序。在T _N2以下，观察到磁弹性耦合以及单轴负热膨胀。