Exploring\nthe Links between Structural Distortions,\nOrbital Ordering, and Multipolar Magnetic Ordering in Double Perovskites\nContaining Re(VI) and Os(VII)

A combination of high-resolution powder diffraction techniques\nand solid-state NMR has been employed to explore the links between\ncrystal structure, orbital ordering, and magnetism in three isostructural\ndouble perovskites containing transition metal ions with a 5d<sup>1</sup> configuration. In Ba<sub>2</sub>ZnReO<sub>6</sub>, both neutron\nand synchrotron X-ray powder diffraction data reveal a cubic-to-tetragonal\ntransition at 23 K that breaks the degeneracy of the t<sub>2g</sub> orbitals and leads to a pattern of orbital ordering that stabilizes\nmagnetic ordering when the sample is cooled below 16 K. Similar behavior\nis observed in Ba<sub>2</sub>MgReO<sub>6</sub>, with an orbital ordering\ntemperature of 33 K and a magnetic ordering temperature of 18 K. Prior\ntheoretical works suggest that the pattern of orbital order seen in\nthe <i>P</i>4<sub>2</sub>/<i>mnm</i> space group\nis needed to stabilize the heavily canted antiferromagnetism of these\ncompounds. Unfortunately, powder diffraction data is not sensitive\nenough to differentiate between the <i>I</i>4/<i>mmm</i> and <i>P</i>4<sub>2</sub>/<i>mnm</i> structural\nmodels, as the distortions are too subtle to be unambiguously identified\nfrom either neutron or synchrotron X-ray powder diffraction methods.\nIn contrast, both diffraction and <sup>7</sup>Li NMR data indicate\nthat Ba<sub>2</sub>LiOsO<sub>6</sub> retains the cubic structure down\nto 1.7 K. The antiferromagnetic ground state and lack of any sign\nof orbital ordering in Ba<sub>2</sub>LiOsO<sub>6</sub> provide compelling\nevidence that the electronically driven tetragonal distortion seen\nin Ba<sub>2</sub>ZnReO<sub>6</sub>, and Ba<sub>2</sub>MgReO<sub>6</sub> is intimately linked to the magnetic ordering seen in those compounds.\nThe absence of magnetic reflections in high intensity neutron powder\ndiffraction data collected on Ba<sub>2</sub>MgReO<sub>6</sub> strongly\nsuggests ordering of multipolar moments on Re(VI), likely ferro-octupolar\nordering.