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.