Eric Quarez

From the mixed valent 6H-Ba3Ru5.5+2NaO9 to the 6H-Ba3(Ru1.69C0.31)(Na0.95Ru0.05)O8.69 oxycarbonate compound

Abstract Ba3Ru2NaO9 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 P63/mmc, Z=2, R1=1.84%, wR2=3.91%. The mean ruthenium valence is 5.5+ for a unique Ru crystallographic site suggesting itinerant electrons within Ru2O9 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 T2=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 CO2−3 anions in the material. Thus, part of the Ru2O9 dimers are replaced by one RuO5 square pyramid and one CO3 group leading to the nominal Ba3(Ru1.69C0.31)(Na0.95Ru0.05)O8.69 formula. The carbonates typical vibration bands have been observed by infrared spectroscopy and clearly distinguished from possible BaCO3 impurity bands. Compared to the ideal 6H-Ba3Ru5.5+2NaO9, 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 RuO5 and (Na/Ru)O6 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 Ba3Ru2NaO9. 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 RuO5 magnetic interaction with the other magnetic moieties. Electron diffraction patterns show a superstructure phenomenon in the (a,b) plane for Ba3Ru2NaO9 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.

Structural investigation of composite phases Ba1 + x [(NaxMn1 – x)O3] with x approx. 2/7, 5/17 and 1/3; exotic Mn4.5+ valence

Abstract Structural models are proposed for three composite compounds of chemical formula Ba1 + x [(NaxMn1 – x)O3] (x approx. 2/7, 5/17 and 1/3) by single crystal X-ray diffraction; superspace formalism is used to obtain an unified description of the three phases. The modulation affecting Ba atoms can be easily designed but the competition “occupational/displacive” modulations relating to the Mn/Na metallic columns were particularly difficult to modelize. However, the large amplitude of the displacive modulation affecting the oxygen atoms (approx. ±0.7 Å) in comparison with that observed for related compounds (approx. ±0.3 Å) makes it a direct consequence of the Na+ alkali insertion inside the trigonal prisms. Owing to this insertion, the Mn atoms exhibit, in the three phases, an “exotic” oxidation state of about +4.5. In addition, even if the sequence between face sharing MnO6 octahedra and NaO6 trigonal prisms can be analytically deduced from the x value, it is clear that the Na/Mn contrasts play in favour of its accurate determination through the XRD single crystal refinement.

Metal Atom Clusters as Building Blocks for Multifunctional Proton-Conducting Materials: Theoretical and Experimental Characterization

The search for new multifunctional materials displaying proton-conducting properties is of paramount necessity for the development of electrochromic devices and supercapacitors as well as for energy conversion and storage. In the present study, proton conductivity is reported for the first time in three molybdenum cluster-based materials: (H)4[Mo6Br6S2(OH)6]-12H2O and (H)2[Mo6X8(OH)6]-12H2O (X = Cl, Br). We show that the self-assembling of the luminescent [Mo6L8i(OH)6a]2-/4- cluster units leads to both luminescence and proton conductivity (σ = 1.4 × 10-4 S·cm-1 in (H)2[Mo6Cl8(OH)6]-12H2O under wet conditions) in the three materials. The latter property results from the strong hydrogen-bond network that develops between the clusters and the water molecules and is magnified by the presence of protons that are statistically shared by apical hydroxyl groups between adjacent clusters. Their role in the proton conduction is highlighted at the molecular scale by ab initio molecular dynamics simulations that demonstrate that concerted proton transfers through the hydrogen-bond network are possible. Furthermore, thermogravimetric analysis also shows the ability of the compounds to accommodate more or less water molecules, which highlights that vehicular (or diffusion) transport probably occurs within the materials. An infrared fingerprint of the mobile protons is finally proposed based on both theoretical and experimental proofs. The present study relies on a synergic computational/experimental approach that can be extended to other proton-conducting materials. It thus paves the way to the design and understanding of new multifunctional proton-conducting materials displaying original and exciting properties.