A 4H-type BaMnO₃ single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P6₃/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO₆ octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at T_N ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P6₃/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO₆ dimers are separated by MnO₆ octahedral layers in the ab plane, forming an -A–B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at T_N1 ≈ 220 K and T_N2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO₃.

中文翻译：

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4H-BaMnO3单晶及其6H型多晶型物的高压合成及磁性

采用浮区法与5 GPa、1023 K高压处理相结合制备了4 H型BaMnO ₃单晶。晶体结晶为六方结构，空间群为P 6 ₃ / mmc，晶格参数a = 5.63723 (5) Å 和c = 9.22355(8) Å。在这种结构中，共面的MnO ₆八面体二聚体通过沿c轴方向的角O原子相互连接，形成-ABAC型4 H排列。发现在T _N处发生长程反铁磁 (AFM) 相变≈ 263 K。当合成压力增加到 20 GPa 时，获得新的多晶相。这种高压相仍然具有六方P 6 ₃ / mmc对称性，但晶格参数变为a = 5.61349(2) Å 和c = 13.66690(9) Å，晶胞体积减少 2.05%。在这个新相中，c轴 MnO ₆二聚体被ab平面中的MnO ₆八面体层隔开，形成 -A-BCACB 型 6 H结构。6 H相在T _N1 ≈ 220 K 和T_N2 ≈ 25 K，分别。根据4 H - 和6 H -BaMnO ₃的详细结构组成讨论了不同的磁性。