Indentation Behavior of Silica Optical Fibers Aged in Hot Water

Vickers indentations were performed on silica optical fibers subjected to water interaction by a hydrothermal treatment. The observation of the Vickers impression provided direct evidence of a piling-up of material of the fiber submitted to the hydrothermal treatment. This indentation behavior is consistent with a structural relaxation promoted by the water and the glass network interaction. It is suggested that the water could act as a modifier since the deformation combines the densification and the shear flow of the material. When the relaxed surface layer is chemically etched, the anomalous behavior of silica under indentation is restored as the deformation is controlled only by densification. The chemical etched samples exhibited an increase in hardness.     

La5Mo6O21: a novel ternary reduced molybdenum oxide containing {MoIV}3 clusters and isolated MoV centres

The crystal structure of La₅Mo₆O₂₁ (penta­lanthanum hexa­molybdenum henicosa­oxide) is made up of Mo₃O₁₃ units containing triangular {Mo^IV}₃ clusters, three distorted Mo^VO₆ octa­hedral units and six inter­stitial La^III atoms. The Mo₃O₁₃ unit consists of three edge‐sharing Mo^IVO₆ units involving Mo—Mo bonding. The three Mo^VO₆ octa­hedra share their corners or edges with each other and with the Mo₃O₁₃ units.     

A consensus view of the temperature dependence of the gas phase reaction: OH + HBr → H2O + Br

The temperature dependence of the rate coefficient for the reaction, OH + HBr has been reinvestigated at low temperatures (T = 48–224 K) by using uniform supersonic flow reactors with laser induced fluorescence detection. This paper presents two forms of global fits: k(T) = 1.11 × 10^?11 (T/298)^?0.91 cm³ s^?1 and k(T) = 1.06 × 10^?11 (T/298)^?1.09 cm³ s^?1, both of which accurately describe the temperature dependence of the rate coefficient for the title reaction within the temperature range 20–350 K. These fits indicate that at temperatures below 200 K, the rate coefficient for this reaction shows inverse temperature dependence, while above 200 K the reaction shows insignificant temperature dependence. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 339–344, 2002     

Enantioselective desymmetrization of meso bicyclic hydrazines: a novel approach to the asymmetric synthesis of polysubstituted amino cyclopentanic cores

Catalytic asymmetric hydroboration can be successfully applied to meso bicyclic hydrazines. The resulting alcohols are of great synthetic interest and can lead in a straightforward manner to cyclopentanic diamino alcohols with good enantiomeric purity.     

Syntheses and structural, physical, and theoretical studies of the novel isostructural Mo9 cluster compounds Ag(2.6)CsMo9Se11, Ag(4.1)ClMo9Se11, and h-Mo9Se11 with tunnel structures

The new isostructural compounds Ag(2.6)CsMo9Se11 (1) and Ag(4.1)ClMo9Se11 (2) were prepared by solid-state reaction in evacuated sealed silica tubes at 1200 degrees C and 860 degrees C, respectively. By topotactic reduction-oxidation reaction of Ag(4.1)ClMo9Se11 with I2, we also obtained the metastable compound h-Mo9Se11 (3). The three compounds crystallize in the hexagonal space group P6(3)/m, Z = 2, (1) a = 10.0472(2) A, c = 11.9548(2), (2) a = 10.0321(2) A, c = 11.8734(2) A, and (3) a = 9.4204(2) A, c = 12.1226(2) A. Their crystal structures were determined from single-crystal X-ray diffraction data and consist of interconnected Mo9Se11 units forming an original and unprecedented three-dimensional framework in which large tunnels are occupied randomly by a part of the Ag+ and the Cl- ions in 2 and the Cs+ ions in 1, the remaining Ag+ in 1 being localized in mirror planes around the 3-fold axis. First-principle calculations allow the understanding of the variation of the atomic distances. Electrical resistivity measurements carried out on single crystals of Ag(2.6)CsMo9Se11 and Ag(4.1)ClMo9Se11 in which the number of electrons per Mo9 cluster is different indicate that the former is semiconducting whereas the latter is semimetallic.     

MTi(0.7)Mo(0.3)Mo(5)O(10) (M = SR, Eu), first evidence of mono- and bicapped bioctahedral Mo(11) and Mo(12) clusters: synthesis, crystal structures, and physical properties

The novel quaternary reduced molybdenum oxides MTi(0.7)Mo(0.3)Mo(5)O(10) (M = Sr, Eu) have been synthesized by solid-state reaction at 1400 degrees C for 48 h in sealed molybdenum crucibles. Their crystal structures were determined on single crystals by X-ray diffraction. Both compounds crystallize in the orthorhombic space group Pbca with 8 formula units per cell and the following lattice parameters: a(Sr) = 9.1085 (7), b(Sr) = 11.418 (1), and c(Sr) = 15.092 (3) A; a(Eu) = 9.1069 (7), b(Eu) = 11.421 (2), and c(Eu) = 15.075 (1) A. The Mo network is dominated by bioctahedral Mo(10) clusters, which coexist randomly with Mo(11) and Mo(12) clusters (monocapped and bicapped Mo(10) clusters). The Mo-Mo distances within the clusters range from 2.62 to 2.92 A and the Mo-O distances from 1.99 to 2.17 A as usually observed in the reduced molybdenum oxides. The Sr(2+) and Eu(2+) ions occupy large cavities, which result from the fusion of two cubooctahedra and thus are surrounded by 11 oxygen atoms. The M-O distances range from 2.50 to 3.23 A for the Sr compound and from 2.49 to 3.24 A for the Eu analogue. Single-crystal resistivity measurements indicate that both materials are poor metals with transitions to semiconducting states below 50 and 40 K and room temperature resistivity values of 9 x 10(-3) and 5 x 10(-3) Omega.cm for the Sr and Eu compounds, respectively. The magnetic susceptibility data indicate paramagnetic behavior due to the Eu(2+) moment at high temperatures for the Eu compound and do not reveal the existence of localized moments on the Mo and Ti sublattice in the Sr compound. An XPS study clearly suggests that the isolated Ti ions are tetravalent. Theoretical considerations preclude the existence of heterometallic Mo-Ti clusters.     

Large specific absorption rates in the magnetic hyperthermia properties of metallic iron nanocubes

We report on the magnetic hyperthermia properties of chemically synthesized ferromagnetic 11 and 16 nm Fe(0) nanoparticles of cubic shape displaying the saturation magnetization of bulk iron. The specific absorption rate measured on 16 nm nanocubes is 1690±160 W/g at 300 kHz and 66 mT. This corresponds to specific losses-per-cycle of 5.6 mJ/g, largely exceeding the ones reported in other systems. A way to quantify the degree of optimization of any system with respect to hyperthermia applications is proposed. Applied here, this method shows that our nanoparticles are not fully optimized, probably due to the strong influence of magnetic interactions on their magnetic response. Once protected from oxidation and further optimized, such nano-objects could constitute efficient magnetic cores for biomedical applications requiring very large heating power.     

Interaction Mechanisms between guaiacols and lignin: the conjugated double bond makes the difference

Lignin is considered to be responsible for a selective sorption of phenolic compounds on wood. In order to investigate the mechanisms involved, two similar guaiacol compounds--only differing by the nature of the para side chain--were adsorbed on oak wood extracted lignin. Vapor sorption-desorption isotherms indicated that about 3.5 wt % of 4-vinylguaiacol is adsorbed near saturation whereas it is only 0.8% for 4-ethylguaiacol. For both compounds, the isotherms displayed a hysteresis though significantly greater for 4-vinylguaiacol. Analyses of the hydroxyl stretching region of FTIR spectra of the lignin/4-ethylguaiacol and lignin/4-vinylguaiacol complexes indicated that physisorption via hydrogen bonds occurs for both sorbates with lignin phenolic hydroxyl groups but which would be more condensed for the former than for the latter. According to NMR spectra, these phenolic hydroxyl groups correspond to non-etherified guaiacyl subunits. In contrast with other para substituents, the conjugated vinyl bond favors not only physisorption but also chemisorption as witnessed by the fact that upon desorption in the vapor phase, even after pumping under dynamic vacuum for several days, about 1 wt % of 4-vinylguaiacol remains adsorbed onto lignin.     

Synthesis, crystal and electronic structures, and magnetic properties of LiLn9Mo16O35 (Ln = La, Ce, Pr, and Nd) compounds containing the original cluster Mo16O36

The new compounds LiLn(9)Mo(16)O(35) (Ln=La, Ce, Pr, and Nd) were synthesized from stoichiometric mixtures of Li(2)MoO(4), Ln(2)O(3), Pr(6)O(11) or CeO(2), MoO(3), and Mo heated at 1600 °C for 48 h in a molybdenum crucible sealed under a low argon pressure. The crystal structure, determined from a single crystal of the Nd member, showed that the main building block is the Mo(16)O(36) unit, the Mo(16) core of which is totally new and results from the fusion of two bioctahedral Mo(10) clusters. It can also be viewed as a fragment of an infinite twin chain of edge-sharing Mo(6) octahedra. The Mo(16)O(36) cluster units share some oxygen atoms to form infinite chains running parallel to the b axis, which are separated by the rare-earth and lithium cations. (7)Li-NMR experiments, carried out at high field on the nonmagnetic LiLa(9)Mo(16)O(35), provided insights into the local environment of the lithium ions. Magnetic susceptibility measurements confirmed the trivalent oxidation state of the magnetic rare-earth cations and indicated the absence of localized moments on the Mo(16) clusters. The electronic structure of the LiLn(9)Mo(16)O(35) compounds was analyzed using molecular and periodic quantum calculations. The study of the molecular orbital diagrams of isolated Mo(16)O(36) models allowed the understanding of this unique metallic architecture. Periodic density functional theory calculations demonstrated that few interactions occur between the Mo(16) clusters, and predicted semiconducting properties for LiLn(9)Mo(16)O(35) as a band gap of 0.57 eV was computed for the lanthanum phase.