André Perrin

New compounds of the type Mo2Re4X8 (M = S, Se) containing octahedral Mo2Re4 clusters

We report the preparation and some properties of new compounds with the composition Mo2Re4X8 (X = S, Se) and Mo4Re2Te8. These compounds are isostructural to Mo6Se8 and present mixed octahedral clusters : Mo2Re4 or Mo4Re2. The solid solutions Mo2Re4S8−xSex (0 ⩽ x ⩽ 8) and Mo2Re4Se8−xTex (0 ⩽ x ⩽ 1.2) are semiconducting, whereas Mo4Re2Te8 is metallic and becomes superconducting at Tc = 3.55 K.

Epitaxial growth of LiNbO3 thin films in a microwave oven

Abstract Oriented LiNbO 3 thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 °C for 10 min. A microstructural study indicated that the films treated at temperatures below 600 °C were homogeneous and dense, and the optical properties confirmed the good quality of these films.

Synthesis and electrical properties of new chalcogenide compounds containing mixed (Mo, Me)6 octahedral clusters (Me = Ru or Rh)

Abstract We report the synthesis and some properties of new compounds and some solid solutions containing mixed octahedral ( Me ) 6 clusters ( Me = Mo, Ru, Rh). The compound Mo 4 Ru 2 Se 8 is semiconducting whereas the limit compounds Mo 4 Ru 2 Te 8 and Mo 4.66 Rh 1.33 Te 8 are metallic. Superconductivity was found around the composition Mo 5 RuTe 8 .

Silver Molybdate and Silver Tungstate Nanocomposites with Enhanced Photoluminescence

Silver molybdate (Ag2MoO4) and silver tungstate (Ag2WO4) nanomaterials were prepared using two complementary methods, microwave assisted hydrothermal synthesis (MAH) (pH 7, 140 °C) and co-precipitation (pH 4, 70 °C), and were then used to prepare two core/shell composites, namely α-Ag2WO4/β-Ag2MoO4 (MAH, pH 4, 140 °C) and β-Ag2MoO4/β-Ag2WO4 (co-precipitation, pH 4, 70 °C). The shape and size of the microcrystals were observed by field emission scanning electron microscopy (FE-SEM), different morphologies such as balls and nanorods. These powders were characterized by X-ray powder diffraction and UV-vis (diffuse reflectance and photoluminescence). X-ray diffraction patterns showed that the Ag2MoO4 samples obtained by the two methods were single-phased and belonged to the β-Ag2MoO4 structure (spinel type). In contrast, the Ag2WO4 obtained in the two syntheses were structurally different: MAH exhibited the well-known tetrameric stable structure β-Ag2WO4, while co-precipitation afforded the metastable β-Ag2WO4 allotrope, coexisting with a weak amount of the α;-phase. The optical gap of β-Ag2WO4 (3.3 eV) was evaluated for the first time. In contrast to β-Ag2MoO4/β-Ag2WO4, the α-Ag2WO4/β-Ag2MoO4 exhibited strongly-enhanced photoluminescence in the low-energy band (650 nm), tentatively explained by the creation of a large density of local defects (distortions) at the core-shell interface, due to the presence of two different types of MOx polyhedra in the two structures.

Octahedral clusters in molybdenum(II) and rhenium(III) chalcohalide chemistry

Abstract A number of molybdenum(II) or rhenium(III) octahedral cluster compounds, mainly in chalcohalide solid state chemistry, are reported. As shown by single-crystal structure determinations, all these compounds contain (Me 6 L 8 i )L 6 a units which progressively condense when halogen ligands are substituted by chalcogen ligands according to Schafers scheme. The physical properties are discussed in relation to the crystal structures and to electronic band structures determined by recent theoretical calculations.

New families of ternary molybdenum (II) chlorides with octahedral Mo6 clusters

A number of new ternary molybdenum +II chlorides have been synthesized by solid state chemistry methods. These compounds belonging to three families −MIIMo6Cl14, MI2Mo6Cl14 and MIMo6Cl13− are characterized by X-ray data and their main structural features are reported. All these compounds are built from (Mo6Cl8)Cl6 units including octahedral Mo6 clusters. In the first family, these units are linked together by M2+ ions in octahedral Cl6 sites whereas in the second one, the cationic sites (M2Cl6) are splitted or distorted. In the third family, the units are linked together along one direction of space by chlorine bridges and the M+ ions lie in quasi tetrahedral sites.

Rhenium octahedral clusters: Characterization of Re6Se4Cl10 and the parent compound Re6Br10

Abstract The first member of the new series Re6Se4+xCl10 ·x−Re6,Se4Cl10 — is described. The main feature of the single-crystal structure determination is the occurrence of octahedral Re6 clusters included in Re6(Se4Cl4)Cl6, units. This insulating material, as well as Re6S4Br10, is quasi-molecular and exhibits some unusual dielectric behaviour.

Evidence for an oxygen uptake effect on the 110 K transition in Bismuth high-Tc superconducting compounds

Abstract The effect of the annealing conditions on the superconductive transitions in BiSrCaCuO samples of various compositions has been studied by a.c. susceptibility. It is confirmed that the annealing conditions are very critical, in particular for the observation of the “110 K” transition. For the nominal composition 1:1:1:2, bulk samples in which the 110 K transition amplitude attains up to 40% of the full amplitude have been obtained. The magnetization of these samples is reported. A thermal treatment under vacuum in mild conditions has a dramatic effect on the amplitude and the onset temperature of the 110 K transition. Initial values are readily restored by a short annealing under oxygen flow at low temperature, giving strong evidence for a reversible oxygen uptake as previously observed in the YBaCuO family.

Photoelectrochemical properties of Re6Se8Cl2 a lamellar transition metal cluster compound

Abstract Photoelectrochemical properties of vapor-grown single crystals Re6Se8Cl2 are presented; this Re6Se8Cl2 is a n-type semiconductor with a direct bandgap of about 1.42eV, corresponding to a probable d-d phototransition. The material is not stable under prolonged illumination in aqueous acidic and basic solutions. Addition of a I−/I−3 redox system in acidic solution increases the photopotential (very strongly) to 490mV, shifts 600mV cathodically the onset of the anodic photocurrent and improves considerably the stability of the Re6Se8Cl2 photoanode. This behavior is analysed in relation to the d band semiconductor character of Re6Se8Cl2.

TiAl-based alloys with nickel

Abstract The alloying potential of nickel as a minor constituent has been investigated in TiAl-based intermetallics with respect to its constitutional ability to provide the necessary microstructures for higher ductility and low fracture toughness. Six alloys (Ti0.60Al0.40)1 − x1Nix1 and (Ti0.56Al0.44)1 − x2Nix2 with x1.2 = 0.01, 0.02 and 0.05 have been studied by means of X-ray diffraction, optical microscopy, EDS and WD-EMP analyses on alloys as-cast and annealed at 900 °C for 240 h under vacuum. The microstructures obtained confirm the phase triangulation at 900 °C near the TiAl boundary. The solubility limit of Ni in the α2-phase is at lower Al/Ni contents than hitherto shown. The Ti, Al-rich vertices of the three-phase equilibrium at 900 °C, α2-Ti3Al(Ni) + γ-TiAl(Ni) + τ2-TiNiAl2, were located at Ti51.5Ni0.7Al47.8 (in at.%) and at Ti62.0Ni0.9Al37.1 respectively. Thus TiAl-based alloys with nickel can only develop a lamellar α2 + γ structure for small amounts of Ni (less than 1 at.% Ni at 900 °C). To compare the effects of Mn and Ni additions on γ-TiAl, self-consistent FLAPW bandstructure calculations were performed for model compounds Ti2XAl and TiXAl2 (X = Mn, Ni). Lattice parameters were determined by minimizing the total energy. From the comparison of the formation energies of the compounds, we found that Ni prefers Ti sites whereas Mn preferably substitutes Al. Taking into account the site preference both ternary additions tend to reduce the c a ratio.