National Institute Of Materials Physics - Romania

It is well-known that the diffusion coefficients of the Cu+ cation in the NaCl and KCl lattices exceed by three or four orders of magnitude the corresponding self-diffusion coefficients in the intrinsic temperature regions. This fast diffusion of the Cu+ has been explained in many papers as an interstitial diffusion although the optical spectra do not confirm the existence of interstitial Cu+. In this paper a new mechanism for fast diffusion is proposed. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability. The main result shows that the off-center position considerably enhances the diffusion. Theoretical diffusion coefficients are obtained by modelling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. The numerical results of the diffusion coefficient computed in this way are in agreement with the experimental data and it is shown that if the impurity is placed dose to the central site the due diffusion coefficient is dose to that for the cationic self-diffusion. The Li+ cation case is dealt with as the case of the Cu+ cation.

A phase analysis of the components resulting from a complex program of reactions for the preparation of ((Bi, Pb)O)(2)-Sr2-xCaxBin-1 FenO3n+1+z with x = 0, x = 0 and n = 1, n = 2, using Fe-57 Mossbauer spectroscopy, was performed. Whereas for the phases with n=1 only relatively slight differences in the values of the Neel temperature between 207 and 250 K were found, the phases with n = 2 presented stronger differences, the magnetic ordering temperatures ranging for different conditions of reactions between 205 and 512 K. The two-phase sample with n=2 ordered magnetically below 60 K. An increase in the duration and/or temperature of reaction resulted in compounds with a high degree of inhomogeneity in the vicinity of the Fe sites leading to a distribution of Fe spin freezing temperatures. All the phases studied with n=1 exhibited a Morin spin reorientation transition, with the spins parallel to the c-axis at low temperatures and perpendicular to it above 100-150 K. In all the samples studied about 4%-11% Fe in the form of high spin Fe4+ was present, the corresponding Mossbauer sextet vanishing at higher temperatures because of faster electron hopping.

The growth and properties of PbS thin films, chemically deposited on bull; lead titano-zirconate ceramics, are investigated. Scanning electron microscopy photographs reveal that the medium size of crystallites is much larger for PbS films grown on poled ferroelectric substrates than for the films grown onto glass or onto unpoled ferroelectric substrates. In the first case the medium size of crystallites is about 1-1.5 mu m while in the second case it is less than 0.3 mu m. Spectral distribution of photoconductivity and thermally stimulated currents (TSC) measurements were performed on all samples. Experimental results indicate that the properties of the PbS films can vary, depending on the substrate nature and in the case of poled ferroelectric substrates, on the polarity. The photoconductive signal is usually larger, the dark resistivity is smaller, and the TSC current value is higher for PbS films deposited on the positive face of the ferroelectric substrate than for those deposited on the negative face. (C) 1995 American Institute of Physics.

Polyaniline (PANI) films were investigated at 20 degrees C as the positive electrodes for Zn/PANI secondary cells containing either 1 M ZnCl2, 0.5 M NH4Cl or 1 M ZnSO4, 0.18 M (NH4)(2)SO4 electrolytes, respectively, at pH=4.5, Cyclic voltammograms showed similar behaviour for the thin PANI films as those in highly acid media. From the discharge curves specific capacities were calculated of 127-130 Ah kg(-1) for the Zn/PANI, Cl- cell and 82-107 Ah kg(-1) for the Zn/PANI, SO42- cell. Charging limit values of 1.48 V for the Zn/PANI, Cl- cell and 1.52 V for the Zn/PANI SO42- cell were obtained in order to prevent the electrochemical inactivation of the polymer. After 20 to 30 cycles the capacity of both cells decreases rapidly and then levels off. The self-discharge process is important only in the first approximately 80 h, after which the open-circuit voltage remains constant. Zn/PANI, Cl- cell showed better electrical characteristics for use as a secondary system.

The particle density fluctuations generated by a weak unidirectional periodic potential in a two-dimensional interacting electron system, in a strong perpendicular magnetic field, are calculated using a local-density description of the exchange-correlation energy for the lowest Landau level. The correlation effects are discussed with respect to previous results obtained in the Hartree and Hartree-Fock approximations. In the spatial regions where the local filling factor is in the range 0.3-0.7, the correlation corrections cancel the exchange contributions, such that the Hartree picture is shown to be qualitatively correct. In these regions, the external potential is strongly screened by the electronic system. In the other spatial regions the density has higher order oscillations with a specific wavelength, which disappear with increasing temperature or the external potential amplitude. Possible experimental evidence of these oscillations in modulated structures is discussed.

The paper presents the experimental results got by adding some inorganic and organic additives to the negative active mass of the lead-sulphuric acid battery plates. The effect of some surface active substances, lignin derivatives,was rendered evident by determining the specific surface of the active masses in their non-formed or formed state, or following some loading-unloading experimental cycles.

The effects of neutron irradiation on the critical temperature and critical current density at 77 K and zero magnetic field of high-temperature superconductors was studied in Li-doped YBa2Cu3O7-x. Damage and thermal neutron reactions with Li produce an insignificant increase of the critical temperature for Li inserted as flame-melted Li2CuO2 while for the others it remains nearly unaffected. The critical transport current density of the sintered YBa2Cu3O7-x remains unaffected for samples with Li2CuO2 and shows a decrease for LiF-doped samples.

Spectral properties of - Delta + V(x), where V(x) lies in a 'neighbourhood' of the periodic case and 'describes various 'models of disorder', are studied. We prove the exponential decay of generalized eigenfunctions corresponding to energies in the resolvent set of the 'unperturbed' periodic Hamiltonian, as well as the stability of the essential spectrum for the 'dislocation disorder' in two dimensions.