Ba 3Ru 2NaO 9 single crystals have been prepared by electrosynthesis in molten NaOH. It adopts a 6H perovskite crystal structure, a=5.8645(6) Å, c=14.440(2) Å, space group P6 3/ mmc, Z=2, R 1=1.84%, wR 2=3.91%. The mean ruthenium valence is 5.5+ for a unique Ru crystallographic site suggesting itinerant electrons within Ru 2O 9 dimers. Previously to our study, a Ru(V)/Ru(VI) charge ordering has been evidenced at 210 K by both single crystal XRD and susceptibility measurements. The resistivity measurement versus temperature performed in the current work shows a brutal R increase at this temperature. Below this temperature, a residual magnetic moment is observed on magnetization plots but has not been observed at 2 K. An additional anomaly is also evidenced on the R versus T plot, at T 2=50 K. It is related to a magnetic transition of questionable origin appearing around the same temperature. Attempts to prepare the title compound by solid state reaction lead to a new related-phase. In fact, combined X-ray and neutron diffraction data unambiguously show the presence of CO 2− 3 anions in the material. Thus, part of the Ru 2O 9 dimers are replaced by one RuO 5 square pyramid and one CO 3 group leading to the nominal Ba 3(Ru 1.69C 0.31)(Na 0.95Ru 0.05)O 8.69 formula. The carbonates typical vibration bands have been observed by infrared spectroscopy and clearly distinguished from possible BaCO 3 impurity bands. Compared to the ideal 6H-Ba 3Ru 5.5+ 2NaO 9, the oxycarbonate main characteristic is the ruthenium +5.28 mean valence. Numerically, such a valence can be obtained considering all the dimeric ruthenium with a +5 oxidation number and the RuO 5 and (Na/Ru)O 6 corner sharing octahedra to be +6. The structure has also been refined by Rietveld analysis of powder neutron diffraction data recorded at 20 K. No structural difference is observed at low temperature. The Ba/Na/Ru oxycarbonate shows sensitive modifications of its physical properties as compared to Ba 3Ru 2NaO 9. Its conductivity obeys an Arrhenius law and no transitions is observed on cooling. The magnetic susceptibility shows a Curie–Weiss behavior until 120 K, afterward, a weak magnetic moment appears and may be due to the RuO 5 magnetic interaction with the other magnetic moieties. Electron diffraction patterns show a superstructure phenomenon in the ( a, b) plane for Ba 3Ru 2NaO 9 while diffuse lines parallel to c ∗ are observed for the oxycarbonate. The HREM contrast has been satisfactorily simulated and explained on the basis of Ba contrast towards lighter species within the 6H blocks but does not allow to distinguish between both compounds.