A. Bulou

Stoichiometry and interface effects on the electronic and optical properties of SiC nanoparticles

The physical properties of nanosized SiC particles are investigated as a function of the C/Si ratios. The synthesis by the laser pyrolysis process monitors the particle stoichiometry by the initial fluxes of the reactants (SiH4 and C2H2) and leads to Si- or C-rich batches. Spectroscopic methods such as EPR and Raman are used to probe the paramagnetic active electronic centres as well as the role played by the external surface composition on the effective vibrational and optical properties of the nanoparticles. Numerical methods are undertaken in order to point out the role played by the carbon or silicon excess on the interface properties. In particular, simulations by quantum chemistry codes within the configuration interaction and the parametric method (PM3) evidence the relevant features of the Raman spectra induced by a suitable distribution of C or Si excess within the SiC nanoparticles.

Polarized Raman study of [N(C3H7)4]2Cd2Cl6 single crystal

Chemical preparation, mid-infrared and Raman spectra of [N(C(3)H(7))(4)](2)Cd(2)Cl(6) are presented. Polarized Raman spectra of oriented single crystals have been recorded in the range 7-3900 cm(-1) under various polarization configurations with regard to the symmetry and the numbers of several band modes observed in the Raman and infrared spectra. The obtained results are consistent with the theoretical predictions based on the infrared and Raman selection rules.

Neutron diffraction study of [Pt(en)2][Pt(en)2I2](ClO4)4 at 20 K: structure and evidence of a new phase transition

The structure at 20 K of fully deuterated [Pt(en)2][Pt(en)2I2](ClO4)4, where en = ethylenediamine, has been determined by neutron diffraction on a single-crystal. It is shown that the symmetry is monoclinic with cell parameters a = 16.650 (7), b = 5.760 (2), c = 14.751 (2) A and β = 99.19 (1)°, that is, with respect to the previously reported room-temperature structures, a doubling of the c parameter is observed. The space group is C2/c and the refinements lead to an R value of 0.0374. All the bond lengths (C—C, C—N, C—D, N—D and Cl—O) and angles are consistent with the expected values and the main difference with respect to the previously reported room-temperature structures results in an ordering of the stacking of the ethylenediamines. The iodines are distributed over two sites, which gives two Pt—I distances, 2.712 (3) A, attributed to the PtIV—I bond, and 3.048 (3) A, attributed to PtII—I. Actually, very weak diffraction peaks that provide evidence for an additional doubling of the a and b parameters are also observed and a space group C2 is therefore proposed. This could result from a very slight ordering of the chains with respect to one another, but, due to the weakness of such signals and the large number of atoms to be considered, no reliable refinement has been obtained in C2. On the other hand, on heating the crystal the diffraction peaks with I odd greatly decrease in intensity between 150 and 160 K, which unambiguously argues for the existence of a phase transition presumably connected to an order—disorder of the ethylenediamines.

Monitoring dehydration of the organic-inorganic [(C3H7)4N][SnCl5(H2O)]·2H2O compound using simultaneous thermal and Raman studies.

In this work we report the experimental studies of the structural phase transition in the [(C3H7)4N]SnCl5(H2O)]·2H2O compound by differential scanning calorimetric (DSC) and Raman spectroscopic. The X-ray powder diffraction study of the [(C3H7)4N][SnCl5(H2O)]·2H2O sample at room temperature showed that this compound is monoclinic and has P121/c1 space group. Differential scanning calorimetric disclosed two types of phase transitions in the temperature range 356-376 (T1) K and at 393K (T2) characterized, by a loss of water molecules and probably a reconstruction of new anionic parts after T2 transition. The Raman scattering spectra recorded at various temperatures in the wavenumber range from 100 to 3800cm(-1) covering the domains of existence of changes in the vicinity of the two phase transitions detected by DSC measurement. A detailed study of the spectral parameters (wave number, reduced intensity and the full width at half maximum) as a function of temperature of a chosen band, associated with (νs(SnO)+νs(SnCl)), based on an order-disorder model allowed us to obtain information relative to the activation energy and correlation length.

Raman scattering study of the upper phase transition in CsScF4

Effects of the phase transition between the high-temperature tetragonal phases of the CsScF4 crystal were studied by Raman scattering. The appearance of new lines including a soft mode was observed below the phase transition point, in complete accordance with the new crystal symmetry. The behavior of the soft mode, which corresponds to in-phase rotations of ScF6 rigid groups around the z-axis (and out-of-phase within perovskite-like layers), resembles that observed earlier in RbAlF4, which suggests a similar mechanism of this transition in both crystals. Copyright

Structural characterization and AC conductivity of bis tetrapropylammonium hexachlorado-dicadmate, [N(C{sub 3}H{sub 7}){sub 4}]{sub 2}Cd{sub 2}Cl{sub 6}

Synthesis, crystal structure, vibrational study, {sup 13}C, {sup 111}Cd CP-MAS-NMR analysis and electrical properties of the compound [N(C{sub 3}H{sub 7}){sub 4}]{sub 2}Cd{sub 2}Cl{sub 6}, are reported. The latter crystallizes in the triclinic system (space group P1-bar, Z = 2) with the following unit cell dimensions: a = 9.530(1) A, b = 11.744(1) A, c = 17.433(1) A, {alpha} = 79.31(1){sup o}, {beta} = 84.00(1){sup o} and {gamma} = 80.32(1){sup o}. Besides, its structure was solved using 6445 independent reflections down to R = 0.037. The atomic arrangement can be described by alternating organic and inorganic layers parallel to the (11-bar 0) plan, made up of tetrapropylammonium groups and Cd{sub 2}Cl{sub 6} dimers, respectively. In crystal structure, the inorganic layer, built up by Cd{sub 2}Cl{sub 6} dimers, is connected to the organic ones through van der Waals interaction in order to build cation-anion-cation cohesion. Impedance spectroscopy study, reported in the sample, reveals that the conduction in the material is due to a hopping process. The temperature and frequency dependence of dielectric constants of the single crystal sample has been investigated to determine some related parameters to the dielectric relaxation.

Vibrational study of Li6P6O18.3H2O and ab initio calculations in P6O18 and P6O18.3H2O.

The infrared and polarized Raman spectra of the trigonal Li(6)P(6)O(18).3H(2)O crystal are reported. The results are analysed using several group theory approaches, in terms of internal and external modes of the highly symmetric P(6)O(18) cyclophosphoric ring and water molecules. Equilibrium geometries and vibrational spectra of P(6)O(18) units, free and in interaction with water molecules (P(6)O(18).3H(2)O) have been determined by ab initio calculations using the basis set 6-31+G(d) of Hartree Fock method. Experimental frequencies and polarisation conditions are remarkably consistent with ab initio calculations. A detailed description of the normal modes of vibration of these systems is presented.