From the mixed valent 6H-Ba_3Ru_2~(5.5+) NaO_9 to the 6H-Ba_3(Ru_(1.69)C_(0.31)(Na_(0.95)Ru_(0.05)O_(8.69) oxycarbonate compound

摘要

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) A, c = 14.440(2) A, 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_3~(2-) 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_(1.69)C_(0.31))(Na_(0.95)Ru_(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_2~(5.5+) NaO_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 low 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.