A combination of high-resolution powder diffraction techniques and solid-state NMR has been employed to explore the links between crystal structure, orbital ordering, and magnetism in three isostructural double perovskites containing transition metal ions with a 5d¹ configuration. In Ba₂ZnReO₆, both neutron and synchrotron X-ray powder diffraction data reveal a cubic-to-tetragonal transition at 23 K that breaks the degeneracy of the t_2g orbitals and leads to a pattern of orbital ordering that stabilizes magnetic ordering when the sample is cooled below 16 K. Similar behavior is observed in Ba₂MgReO₆, with an orbital ordering temperature of 33 K and a magnetic ordering temperature of 18 K. Prior theoretical works suggest that the pattern of orbital order seen in the P4₂/mnm space group is needed to stabilize the heavily canted antiferromagnetism of these compounds. Unfortunately, powder diffraction data is not sensitive enough to differentiate between the I4/mmm and P4₂/mnm structural models, as the distortions are too subtle to be unambiguously identified from either neutron or synchrotron X-ray powder diffraction methods. In contrast, both diffraction and ⁷Li NMR data indicate that Ba₂LiOsO₆ retains the cubic structure down to 1.7 K. The antiferromagnetic ground state and lack of any sign of orbital ordering in Ba₂LiOsO₆ provide compelling evidence that the electronically driven tetragonal distortion seen in Ba₂ZnReO₆, and Ba₂MgReO₆ is intimately linked to the magnetic ordering seen in those compounds. The absence of magnetic reflections in high intensity neutron powder diffraction data collected on Ba₂MgReO₆ strongly suggests ordering of multipolar moments on Re(VI), likely ferro-octupolar ordering.

中文翻译：

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探索包含 re（VI） 和 Os（VII） 的双钙钛矿中结构畸变、轨道排序和多极磁排序之间的联系


高分辨率粉末衍射技术和固体核磁共振相结合，探索了三个包含5d¹构型过渡金属离子的等构双钙钛矿中晶体结构、轨道排序和磁性之间的联系。在 Ba₂ZnReO₆ 中，中子和同步加速器 X 射线粉末衍射数据都揭示了 23 K 处的立方到四方转变，这打破了 t_2g 轨道的简并性，并导致轨道有序模式，当样品冷却到 16 K 以下时，该模式稳定了磁有序。，轨道有序温度为 33 K，磁有序温度为 18 K。先前的理论工作表明，需要 P4₂/mnm 空间群中看到的轨道有序模式来稳定这些化合物的严重倾斜反铁磁性。遗憾的是，粉末衍射数据不够敏感，无法区分 I4/mmm 和 P4₂/mnm 结构模型，因为变形太细微，无法从中子或同步加速器 X 射线粉末衍射方法中明确识别。相比之下，衍射和 ⁷Li NMR 数据都表明，Ba₂LiOsO₆ 在低至 1.7 K 的温度下保留了立方结构。Ba₂LiOsO₆ 中的反铁磁基态和没有任何轨道有序迹象提供了令人信服的证据，证明在 Ba₂ZnReO₆ 和 Ba₂MgReO₆ 中看到的电子驱动四方畸变与这些化合物中的磁有序密切相关。 在 Ba₂MgReO₆ 上收集的高强度中子粉衍射数据中没有磁反射，这强烈表明 Re（VI） 上的多极矩有序，可能是铁-八极有序。