Exploring the Links between Structural Distortions, Orbital Ordering, and Multipolar Magnetic Ordering in Double Perovskites Containing Re(VI) and Os(VII)

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 t2g 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.

本研究结合高分辨粉末衍射技术与固态核磁共振（solid-state NMR），对三类具有同构结构、含5d¹电子构型过渡金属离子的双钙钛矿（double perovskites）展开研究，旨在厘清其晶体结构、轨道有序化与磁学性质间的内在关联。在Ba₂ZnReO₆中，中子衍射与同步辐射X射线粉末衍射数据均显示，其在23 K时发生立方-四方相变；该相变打破了t₂g轨道（t2g orbitals）的简并性，形成特定轨道有序模式，当样品降温至16 K以下时，该有序模式可稳定磁有序结构。Ba₂MgReO₆表现出相似的行为，其轨道有序化温度为33 K，磁有序化温度为18 K。既往理论研究表明，P42/mnm空间群下的轨道有序模式，是稳定该类化合物强倾斜反铁磁性的必要条件。但遗憾的是，粉末衍射数据的灵敏度不足以区分I4/mmm与P42/mnm两种结构模型；因二者晶格畸变过于微弱，无论是中子衍射还是同步辐射X射线粉末衍射方法，均无法对其进行明确指认。与之形成鲜明对比的是，衍射与⁷Li核磁共振（⁷Li NMR）数据均显示，Ba₂LiOsO₆在降温至1.7 K时仍保持立方结构。Ba₂LiOsO₆具备反铁磁性基态且未表现出任何轨道有序迹象，这一特性为Ba₂ZnReO₆与Ba₂MgReO₆中由电子诱导的四方晶格畸变，与这两种化合物的磁有序存在密切关联提供了有力证据。针对Ba₂MgReO₆采集的高强度中子粉末衍射数据中未观测到磁衍射峰，该结果强烈表明，六价铼（Re(VI)）位点存在多极矩有序，极有可能为铁八极有序（ferro-octupolar ordering）。