Structure determination of a brownmillerite Ca2Co2O5 thin film by Precession Electron Diffraction

Calcium cobaltite thin films with a ratio Ca/Co=1 were grown on (101)-NdGaO3 substrate by the pulsed laser deposition technique. The structure of the deposited metastable phase is solved using a precession electron diffraction 3D dataset recorded from a cross-sectional sample. It is shown that an ordered oxygen-deficient Ca2Co2O5+d perovskite of the brownmillerite-type with lattice parameters a= 0.546nm, b=1.488nm and c=0.546nm (SG: Ibm2) has been stabilized using the substrate induced strain. The structure and microstructure of this metastable cobaltite is further discussed and compared to related bulk materials based on our transmission electron microscopy investigationsComment: 13 pages, 10 figures, 2 tables, accepted in Phys. Rev.

A sulfur-rich pi-electron acceptor derived from 5,5 '-bithiazolidinylidene: charge-transfer complex vs. charge-transfer salt

International audienceNovel pi-electron acceptors are still highly desirable for the formation of conducting salts or as n-dopable semiconductors. We describe here two synthetic approaches to substitute a dicyanovinylidene group, C=C(CN)(2) to a thioketone (C=S) in the recently described DEBTTT acceptor where DEBTTT stands for (E)-3,3'-diethyl-5,5'-bithiazolidinylidene-2,4,2',4'-tetrathione. These electron withdrawing groups enhance the electron accepting ability as demonstrated through electrochemical investigations, without hindering the formation of short intra-and intermolecular S center dot center dot center dot S contacts in the solid state. Association of this acceptor 1 with tetramethyltetrathiafulvalene (TMTTF) and decamethylferrocene (Fe(Cp*)(2)) afforded 1 : 1 adducts which were analyzed by single crystal X-ray diffraction. Combined with vibrational and magnetic properties, it appears that [TMTTF][1] behaves as a neutral charge-transfer complex while [Fe(Cp*)(2)][1] is an ionic salt. The concentration of the spin density on the exocyclic sulfur atoms in 1(-center dot) favors the setting of direct anti-ferromagnetic interactions in [Fe(Cp*)(2)][1

Thermal and near-infrared light induced spin crossover in a mononuclear iron( ii ) complex with a tetrathiafulvalene-fused dipyridophenazine ligand

A mononuclear Fe(II) complex involving a tetrathiafulvalene-based ligand exhibits thermal spin-crossover (around 143 K) with pronounced hysteresis behaviour (48 K). The chromophoric and π-extended ligand allows Near-Infrared (NIR) sensitization for the light-induced excited spin-state trapping (LIESST) with T(LIESST) = 90 K

Dramatic Influence of A-Site Nonstoichiometry on the Electrical Conductivity and Conduction Mechanisms in the Perovskite Oxide Na0.5Bi0.5TiO3

Recently, there has been considerable interest in the perovskite phase Na0.5Bi0.5TiO3 (NBT) as a promising lead-free piezoelectric material. Here we report low levels of Na nonstoichiometry (±2 atom % on the A-site) in the nominal starting composition of NBT ceramics can lead to dramatic changes in the magnitude of the bulk (grain) conductivity (σb) and the conduction mechanism(s). Nominal starting compositions with Na excess exhibit high levels of oxide-ion conduction with σb ∼ 2.2 mS cm–1 at 600 °C and an activation energy (Ea) < 1 eV whereas those with Na deficiency are dielectrics based on intrinsic electronic conduction across the band gap with σb ∼ 1.6 μS cm–1 at 600 °C and Ea ∼ 1.7 eV. Drying of reagents, especially Na2CO3, changes the starting stoichiometry slightly due to a small amount of adsorbed moisture in the raw materials but influences significantly the electrical properties. This demonstrates the bulk electrical properties of NBT to be highly sensitive to low levels of A-site nonstoichiometry and reveals how to fine-tune the nominal starting composition of NBT ceramics to suppress leakage conductivity for piezoelectric and high temperature capacitor applications

Growth of single crystals in the (Na1/2Bi1/2)TiO3-(Sr1-xCax)TiO3 system by solid state crystal growth

Ceramics based on (Na1/2B1/2)TiO3 are promising candidates for actuator applications because of large strains generated by an electric field-induced phase transition. For example, the (1-x)(Na1/2Bi1/2)TiO3-xSrTiO(3) system exhibits a morphotropic phase boundary at x = 0.2-0.3, leading to high values of inverse piezoelectric constant d*(33), which can be further improved by the use of single crystals. In our previous work, single crystals of (Na1/2B1/2)TiO3-SrTiO3 and (Na1/2B1/2)TiO3-CaTiO3 were grown by the solid state crystal growth technique. Growth in the (Na1/2B1/2)TiO3-SrTiO3 system was sluggish whereas the (Na1/2B1/2)TiO3-CaTiO3 single crystals grew well. In the present work, 0.8(Na1/2Bi1/2)TiO3-0.2(Sr1-xCax)TiO3 single crystals (with x = 0.0, 0.1, 0.2, 0.3, 0.4) were produced by the solid state crystal growth technique in an attempt to improve crystal growth rate. The dependence of mean matrix grain size, single crystal growth distance, and electrical properties on the Ca concentration was investigated in detail. These investigations indicated that at x = 0.3 the matrix grain growth was suppressed and the driving force for single crystal growth was enhanced. Replacing Sr with Ca increased the shoulder temperature T-s and temperature of maximum relative permittivity T-max, causing a decrease in inverse piezoelectric properties and a change from normal to incipient ferroelectric behavior

Stability of NdBaCo2−xMnxO5+δ (x = 0, 0.5) layered perovskites under humid conditions investigated by high-temperature in situ neutron powder diffraction

The double perovskites NdBaCo2−xMnxO5+δ (x = 0 and 0.5) were investigated using in situ high temperature neutron powder diffraction in dry argon and wet atmospheres (40% D2O/argon and 40% D2O/air) in order to assess their stability as cathodes in proton conducting fuel cells. The x = 0 oxide loses oxygen on heating in dry argon at T > 400 °C and exhibits an oxygen vacancy order–disorder transition as evidenced by the orthorhombic Pmmm to tetragonal P4/mmm transition. Refinement of site occupancy factors suggests that the oxygen vacancies mainly form in the Nd layers and to a lesser extent at the equatorial positions of the transition metal polyhedra; at 800 °C, δ ∼ 0. When the gas was changed to wet argon at 800 °C and the sample cooled to 260 °C, no structural modification or change in the oxygen content was detected and no impurity phases formed, highlighting the excellent stability of the sample in wet atmospheres. On switching the gas to wet air at 260 °C, thermal analysis and neutron powder diffraction data together reveal that the sample intercalates mainly oxygen rather than proton defects within a two-phase process involving two orthorhombic phases, reflecting the symmetry of the reduced and oxidised materials. On heating, the sample transforms at T ≥ 600 °C to a single tetragonal phase whose symmetry is retained up to 800 °C and on subsequent cooling. The x = 0.5 material prepared in argon adopted a tetragonal P4/mmm structure at RT with δ ∼ 0. Its symmetry remains tetragonal on heating/cooling in wet argon. On changing the gas to wet air at 260 °C, it takes up oxygen via a two-phase process involving two tetragonal phases. Since fast oxidation is the main process that fills the oxygen vacancies of these double perovskites in wet air, a large oxygen deficiency seems to be not the only requirement for effective proton incorporation in this family of materials with basic characteristics

The structuraloriginoftheantiferroelectricpropertiesandrelaxorbehavior of Na0.5Bi0.5TiO3

International audienceThis workpresentsastudyofNa0.5Bi0.5TiO3 (NBT) bytransmissionelectronmicroscopyinthe20–370 1C temperaturerange.Aneworthorhombicintermediatephasebetweentherhombohedralandthe tetragonalphasesisproposedtoaccountfortheoccurrenceof 12 (oee) superstructurespots.Thephase transition fromtherhombohedraltotheorthorhombicphaseoccursviaamodulatedphaseformedby rhombohedral blocksandorthorhombicsheets.Itisshownthattheselatterrepresentrhombohedral (010)twinplanes.Themodulatedphaseisproposedtoexplaintheantiferroelectricandrelaxor behaviorsofNBT

Application of the Curtius rearrangement to the synthesis of 1′-aminoferrocene-1-carboxylic acid derivatives

International audienceThe shortest synthesis of N-protected 1′-aminoferrocene-1-carboxylic acid from readily available ferrocene-1,1′-dicarboxylic acid is reported. 1′-Azidocarbonylferrocene-1-carboxylic acid was first obtained by reaction of the latter with diphenylphosphoryl azide. It was then converted into four amino acids by a Curtius rearrangement conducted in the presence of tert-butanol, benzyl alcohol, 9-fluorenemethanol or allyl alcohol. The benzyl and allyl carbamate derivatives are reported and characterized for the first time. The four corresponding new succinimidyl activated esters were also prepared and their usefulness was demonstrated in peptide coupling. Various structures were elucidated by X-ray crystallography, including 1′-azidocarbonylferrocene-1-carboxylic acid and 1,1′-diazidocarbonylferrocene. © 2018 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique

Helically chiral functionalized [6]helicene Synthesis, optical resolution, and photophysical properties

International audienceA short and efficient synthetic pathway leading to a new chiral π-conjugated system is reported. The X-ray structure of the target compound was determined and showed a helical conformation. Its resolution was successfully accomplished, leading to two enantiomers in high optical purity, and their chiroptical properties were examined experimentally. The photophysical properties of the organic material were also evaluated, showing an emission in the visible region, and HOMO and LUMO levels have been estimated experimentally, demonstrating an electrochemical band gap of 2.37 eV. © 2018 Académie des science

Ferroceneboronic Acid and Derivatives Synthesis, Structure, Electronic Properties, and Reactivity in Directed C-H Bond Activation

International audienceKnown since 1959, ferroceneboronic acid (1) and its derivatives have been mainly used as polyol sensors and in cross-coupling reactions. However, a literature survey revealed that there is not a paper describing the full characterization of ferroceneboronic acid derivatives and that useful boron protecting groups have not been studied in the ferrocene series. Here, we present an optimized multigram-scale synthesis of the known ferroceneboronic acid (1) using a phase-switch purification process. It was furthermore functionalized to reach the diaminonaphthalene (FcB(dan), 15), anthranilamides (FcB(aam), 16; FcB(Me-aam), 17 and 18), potassium trifluoroborate (FcBF3K, 19), triolborate (FcB(triolborate), 20) and N-methyliminodiacetic acid (FcB(MIDA), 21) derivatives. Their structures were unambiguously assigned by NMR and X-ray analysis, and the data collected provided a general overview of the electronic and structural features of these compounds. From the data obtained, B(dan) and B(amm) were classified as electron-withdrawing groups, whereas trifluoroborate and triolborate behave as electron-donating groups. We report the first catalytic silylation of ferrocene C-H bonds to access di- and trisubstituted derivatives. Catalytic borylation was also attempted, highlighting a switch in regioselectivity that was unambiguously assigned by X-ray analysis