Na27Ru14O48: A new mixed-valence sodium ruthenate with magnetic heptameric plaquettes

Na27Ru14O48 has been synthesized in air at 700 °C. The composition and crystal structure of the phase were determined by single crystal X-ray diffraction. The triclinic crystal structure contains isolated planar Ru7O24 plaquettes made from seven edge-sharing RuO6 octahedra. The complex Na:Ru ratio is a result of tilting of the plaquettes to disrupt the packing of nominally hexagonal close packed planes made of Na ions and RuO6 octrahedra. Resistivity measurements show that the material is semiconducting with an activation energy of 0.53 eV. The observed magnetic moment of 3.11 μB per Ru is lower than the expected spin only value, but is within the range seen in other compounds and is too large to indicate that the fundamental magnetic entities are the isolated Ru7O24 plaquettes. Small, reproducible deviations in the Curie–Weiss behavior occur below 200 K and the onset of a broad magnetic transition is seen between 40 and 32 K.