| Abstract
| - Two new mixed-valent triple perovskites, Ba3MRu2O9 (M = Li, Na), were grown from reactivehydroxide fluxes. They crystallize in the hexagonal space group P63/mmc, where Ru(V) and Ru(VI) aredisordered on only one crystallographic site. Upon cooling, single crystals of Ba3NaRu2O9 undergo a complexsymmetry-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 ratherabrupt decrease in the magnetic susceptibility at 210 K followed by a steady decrease in the susceptibilitywith decreasing temperature. Interestingly, the lithium analogue does not display any structural transitiondown to 100 K. The structural transition in Ba3NaRu2O9 generates three crystallographically unique Rusites 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 transitionalso 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.
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