Publications

The theoretical treatment of one-phonon thermal diffuse scattering (TDS) in single-crystal neutron diffraction at fixed incident wavelength is reanalysed in the light of the analysis given by Popa & Willis [Acta Cryst. (1994), A50, 57-63; Acta Cryst. (1997), A53, 537-545] for the time-of-flight method. Isotropic propagation of sound with different velocities for the longitudinal and transverse modes is assumed. As in time-of-flight diffraction. there exists, for certain scanning variables, a forbidden range in the one-phonon TDS of slower-than-sound neutrons, and this permits the determination of the sound velocity in the crystal. A fast algorithm is given for the TDS correction of neutron diffraction data collected at a fixed wavelength: this algorithm is similar to that reported earlier for the time-of-flight case.

The L alpha and L beta spectra of tungsten were measured using a high-resolution single-crystal x-ray spectrometer. The observed spectra were fitted into Lorentzians. The fit residuals of the L alpha and L beta spectra indicate the presence of satellites in the vicinity of each spectrum, which are originated from Coster-Kronig transitions. Linewidths, energies, and intensities were estimated for each diagram line and compared with previous data. The L beta(2) satellite line observed by Salgueiro er al. [J. Phys. (Paris), Colloq 48, C9-609 (1987)] was confirmed on the high energy side of the L beta(2) line. The origin of this satellite is discussed experimentally and theoretically. [S1050-2947(98)02011-3].

We measure the azimuthal anchoring energy of the nematic 5CB on sinusoidal holographic unidimensional and bidimensional gratings. We find that the anchoring strength is almost the same for all of the gratings and up to one order of magnitude stronger than expected by the topographic mechanism proposed by Berreman. We observe strong memory effects which dominate both the elastic anchoring and the easy axis gliding. We show that these memory effects limit from below the anchoring energy and are the main obstacle in the realization of substrates with weak anchoring conditions.

Employing X-ray scattering, the phase morphology and the miscibility of two mixtures formed from acrylonitrile-vinylacetate-alpha-methylstyrene copolymer with both styrene-acrylonitrile copolymer and cellulose acetate have been studied. The degrees of crystallinity, determined by X-ray diffraction, revealed the preponderant amorphous character of these mixtures. By small angle scattering of synchrotron radiation, imposing the miscibility condition for Flory-Huggins interaction energy parameter, the "miscibility window" for all mixtures were determined.

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and diskshape) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4; 7.4; 0.2 and 0.12 mu m, respectively) were determined by transmission electron microscopy (TEM) combined with electron diffraction. The haematite samples were studied by transmission Mossbauer spectroscopy in the temperature range 4.2-300 K. In all cases, a weak ferromagnetic phase (WF) was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic phase (Ali) below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology. (C) 1998 Elsevier Science S.A. All rights reserved.

The modification of the ideal lattice parameter in porous silicon has been determined by X-ray diffraction. A contraction effect has been detected in several samples. There was shown that the position of the photoluminescence band is related to the modification of the lattice parameter. The quantum theory of confinement in wires was used for the interpretation of the experimental data.

A quantum confinement model is proposed to explain the electrical transport properties in fresh and stored porous silicon (PS) films. In the present paper, the model is verified for the temperature dependence of the dark current. The studied samples are Formed mainly by a network of nanowires, suggesting the separation of the electron Hamiltonian into a longitudinal and a transversal part. The last one can be well described by a two-dimensional (2D) cylindrical infinite quantum well? whose levels determine the dark current activation energies. The storage oxidation induces modifications both in the number and the values of the activation energies, which are in excellent agreement with our model. (C) 1998 Elsevier Science Ltd. All rights reserved.

The reversible photostructural effects observed in the amorphous thin films and in chalcogenide glasses based on arsenic have been explained with the help of computer relaxed models. Two photostructural mechanisms have been established: the incorporation of the chalcogenide molecular fraction into the disordered layers and the change of the relative positions of the atoms in the layers without destroying the bonding network. In the frame of this atomic scale mechanisms it is possible to explain the reversible photodarkening and the photoanisotropy observed in the chalcogenide thin Alms and bulk glasses.

Various aqueous systems produced by different technologies can contain Mn(II) in the concentration range of 50 divided by 1000 mg x l(-1), which represents a pollution source but also can be used for the separation-recovery of useful compounds. The present paper presents the results of the researches concerning Mn(II) recovery from "model" and real aqueous systems by the precipitate flotation method, dissolved air flotation (DAF) variant, with izoalkylcarboxillic acid as collector. The following influencing factors were studied: pH, collector concentration, metallic ion concentration, necessary gas flow and foam processing with Mn(II) recovery as oxide. The separation mechanism by the collector-Mn(II) interaction at optimal conditions is also studied. Therefore, a correlation of chemical analysis results with electronic and vibrational spectra and thermal analysis is made. The results of the experimental data plead for the recovery separation of Mn(II) from aqueous systems and its recovery as oxide after thermal decomposition of the species contained in the foam. (C) 1998 Elsevier Science Ltd. All rights reserved.

An analytical formula for the optical charging spectroscopy (OCS) discharging current in the case of homogeneous semiconductors is deduced. The computation shows that the OCS current is proportional with the square of the initially trapped carrier concentrations. This result leads to a higher sensitivity in case of the OCS method compared with the thermally stimulated current method. The OCS current formula obtained for one trapping level is generalized in the case of many trapping levels. From the generalized formula it can be seen that the OCS current can change the sign if both electron and hole traps exist in the sample. (C) 1998 American Institute of Physics.