Novel Mixed-Valent (V/VI) Triple Perovskite Ruthenates: Observation of a Complex Low-Temperature Structural and Magnetic Transition

Two new mixed-valent triple perovskites, Ba3MRu2O9 (M = Li, Na), were grown from reactive
hydroxide fluxes. They crystallize in the hexagonal space group P63/mmc, where Ru(V) and Ru(VI) are
disordered on only one crystallographic site. Upon cooling, single crystals of Ba3NaRu2O9 undergo a complex
symmetry-breaking structural transition at ca. 225 K from room-temperature hexagonal symmetry to a low-temperature orthorhombic symmetry, space group Cmcm. Accompanying this structural transition is a rather
abrupt decrease in the magnetic susceptibility at 210 K followed by a steady decrease in the susceptibility
with decreasing temperature. Interestingly, the lithium analogue does not display any structural transition
down to 100 K. The structural transition in Ba3NaRu2O9 generates three crystallographically unique Ru
sites in the low-temperature structure as compared to only one distinct site in the room-temperature structure.
On the basis of an analysis of the Ru−Ru distances in the face-sharing bi-octahedra, the structural transition
also appears to involve charge ordering of Ru(V) and Ru(VI), causing all Ru(V) to occupy one set of bi-octahedra and all Ru(VI) to occupy another set.