Smart control of monodisperse Stöber silica particles: effect of reactant addition rate on growth process

Control over the synthesis of monodisperse silica particles up to mesoscopic scale is generally made difficult due to intrinsic limitation to submicrometric dimensions and secondary nucleation in seeded experiments. To investigate this issue and overcome these difficulties, we have implemented single step processing by quantifying the effects of the progressive addition of a diluted tetraethyl orthosilicate solution in ethanol on the size and monodispersity of silica particles. Contrary to particles grown in seeded polymerization, monodisperse particles with size up to 2 microm were synthesized. Moreover, the particles exhibit a final diameter (d(f)), which varies with V(-1/3) over more than 2 orders of magnitude in rate of addition (V). On the basis of a kinetic study in the presence of addition showing that particle growth is limited by the diffusion of monomer species, we developed a diffusion-limited growth model to theoretically explain the observed d(f)(V) behavior and quantitatively retrieve the measured amplitude and exponent. Using a single parameter procedure, we can therefore predict and generate in the room temperature range, monodisperse particles of a targeted size by simply adjusting the rate of addition.     

Crystal structure and thermal expansion of the low- and high-temperature forms of BaM(PO4)2 compounds (M=Ti, Zr, Hf and Sn)

The crystal structure of β-BaZr(PO₄)₂, archetype of the high-temperature forms of BaM(PO₄)₂ phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar α-structure into a trigonal one (S.G. ) through a simple mechanism involving the unfolding of the layers. The thermal expansion is very anisotropic (e.g., −4.1<α_i<34.0×10⁻⁶ K⁻¹ in the case of α-BaZr(PO₄)₂) and quite different in the two forms, as a consequence of symmetry. It stems from a complex combination of several mechanisms, involving bridging oxygen rocking in M-O-P linkages, and “bond thermal expansion”.     

Self-irradiation induced structural changes in the transplutonium pyrochlores An2Zr2O7 (An=Am, Cf)

We have pursued the fundamental chemistry of actinide pyrochlore oxides, An₂Zr₂O₇ (An=Am, Cm, Bk, and Cf), using X-ray diffraction as well as optical spectroscopy. One recent facet of our studies has been to observe the structural changes of these materials under self-irradiation as a function of time. It has been reported that both titanate and silicate materials transform from a crystalline to an amorphous state under irradiation. With the Zr-based actinide pyrochlores studied here, we have observed a phase change from a pyrochlore structure to a fluorite-type structure with the retention of crystallinity. We focus here on the impact of α-radiation (²⁴³Am and ²⁴⁹Cf), rather than that from neutrons (²⁴⁸Cm) or β-radiation (²⁴⁹Bk), on the An₂Zr₂O₇ pyrochlore structures. As a result of this phase change, the local coordination environments of both the actinide and zirconium atoms are altered. We consider a defect/ion deficiency driven mechanism and also address the occurrence of oxidation of the trivalent actinides during the self-irradiation process as being potential mechanisms responsible for the observed phase change.     

Triclinic-cubic phase transition and negative expansion in the actinide IV (Th, U, Np, Pu) diphosphates

The AnP(2)O(7) diphosphates (An = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the An(IV) cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and (31)P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP(2)O(7). Although they adopt at high temperature the same cubic archetypal cell as the other known MP(2)O(7) diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P(2)O(7) units, as shown using high-resolution synchrotron diffraction for UP(2)O(7). The uncommon triclinic-cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of An(IV). The conflicting effects which control the thermal variations of the P-O-P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller M(IV) cations in which the two phenomena coincide. As shown by various approaches, the P-O(b)-P linkage remains bent in the cubic form.     

The chemistry of the phosphates of barium and tetravalent cations in the 1:1 stoichiometry

The chemistry of phosphates of barium and tetravalent cations [BaM^IV(PO₄)₂] is reviewed. Such phosphates crystallise in the C2/m space group for M^IV=Ti, Zr, Hf, Ge, Sn, and Mo, and in the P2₁/n space group for BaTh(PO₄)₂. The existence of BaM^IV(PO₄)₂ in which M^IV=Pb, Ce, and U is further evaluated. Several aspects, such as phase transitions in the compounds with yavapaiite structure, solid solutions of BaM^IV(PO₄)₂ compounds and practical applications are briefly discussed.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

Lignans from Arnica species

From four Arnica species (A. angustifolia Vahl ssp. attenuata (Greene) Maguire, A. lonchophylla Greene ssp. lonchophylla Maguire (flowerheads), A. chamissonis Less. ssp. foliosa (Nutt.) Maguire, A. montana L. (roots and rhizomes)) a total of twelve lignans of the furofuran-, dibenzylbutyrolactone- and dibenzylbutyrolactol-type were isolated. No report on lignans as constituents of Arnica species exists so far. Besides the known pinoresinol, epipinoresinol, phillygenin, matairesinol, nortrachelogenin and nortracheloside, six dibenzylbutyrolactol derivatives with different stereochemistry and substitution at C-9 were isolated and their structures elucidated by NMR spectroscopic and mass spectral analysis.     

Joint Raman spectroscopic and quantum chemical analysis of the vibrational features of Cs2RuO4

The Raman spectroscopic characterization of the orthorhombic phase of Cs₂RuO₄ was carried out by means of group theory and quantum chemical analysis. Multiple models based on ruthenate (VI+) tetrahedra were tested, and characterization of all the active Raman modes was achieved. A comparison of Raman spectra of Cs₂RuO₄, Cs₂MoO₄, and Cs₂WO₄ was also performed. Raman laser heating induced a phase transition from an ordered to a disordered structure. The temperature‐phase transition was calculated from the anti‐Stokes/Stokes ratio and compared with the ones measured at macroscopic scale. The phase transition is connected with tilting and/or rotations of RuO₄ tetrahedra, which lead to a disorder at the RuO₄ sites. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd.     

Photoluminescence and Raman studies of Sm3+ and Nd3+ ions in zirconia matrices: example of energy transfer and host-guest interactions

Photoluminescence and Raman studies on Sm(3+)- and Nd(3+)-doped zirconia are reported. The Raman studies indicate that the monoclinic (m) phase dominates up to a 10 at.% lanthanide level, while stabilization of the cubic phase is attained at approximately 20 and approximately 25 at.% of Sm(3+) and Nd(3+), respectively. Both systems are strongly luminescent under photo-excitation. The emission spectrum at 77 K of the ZrO(2):Sm(3+) system consists of a broad band at 505 nm, that corresponds to the zirconia matrix. At room temperature the band maximum blue-shifts to 490 nm. Sharper bands corresponding to f-f transitions within the Sm(3+)ion are also exhibited in the longer wavelength region of the spectrum. Exclusive excitation of the zirconia matrix provides sensitized emission from the acceptor Sm(3+) ion. The excitation profile is dominated by a broad band at 325 nm when monitored either at the zirconia or at one of the Sm(3+) emissions. A spectral overlap between the 6H(5/2)-->(4)G(7/2) absorption of the Sm(3+) ion with the zirconia emission leads to an efficient energy transfer process in the systems. Multiple facets of the spectral behavior of the Sm(3+) or Nd(3+) in the zirconia matrices, as well as the effects of compositions on the emission and Raman properties of the materials, and the role of defect centers in photoluminescence and the energy transfer processes are discussed.