National Institute Of Materials Physics - Romania

Models for Al-TM (TM is transition metal) quasicrystals consisting of networks of icosahedral units have the merit to be specific about the transition metal surrounding. Experimental arguments are brought for the importance of local stable Al(12)TM clusters in the formation of disorder-free icosahedral phases.

The intermediate-range order (IRO) in silica gels is investigated by X-ray diffraction and infrared absorption. The IRO degradation in fresh gels with respect to v-SiO2 and its recovery upon annealing are quantitatively characterized.

Thermal analysis of a pyroelectric bimorph structure, performed for a sinusoidally modulate heat flow, reveals the possibility to obtain a larger signal than in the case of a homogenous pyroelectric structure. The ratio between the signal generated from a bimorph structure and the signal generated from a homogenous structure, in the same conditions, depends on the material properties and on the modulation frequency. This ratio, computed for a particular bimorph, is up to four at 1000 Hz.

Ti nitride films were deposited by DC magnetron sputtering on HSS and Si substrates. X-ray diffraction showed the formation of single-phase delta-TiN. (111) texture was found in most films, as well as stacking disorder on (111) planes, both correlated with deposition parameters, including the degree of plasma ionization. Evidence was found for plastic deformation relaxing film stress, The structure data are correlated with film microstructure and microhardness.

The temperature dependence of the electrical resistance R(T) for RF sputtered Ni-100-x-Si-x thin films (33 less than or equal to x less than or equal to 56 at%, as determined by Rutherford backscattering spectroscopy) was measured between -190 degrees C and the annealing temperature T, (T-a max = 300 degrees C). Structure investigations by X-ray diffraction revealed the preferential formation of silicide phases in as-deposited films as dependent on composition: gamma-Ni5Si2 (for x = 33), delta-Ni2Si (37 less than or equal to x less than or equal to 40) and zeta-NiSi2 (for x = 56). For intermediary compositions, amorphous alloys are formed. Annealing at 200 and 300 degrees C induces crystallization of the amorphous films into the gamma-Ni5Si2 phase (with zeta-NiSi2 admixture). The preferential formation of silicide phases is discussed taking into account the phenomenological rule of ''effective formation heat''. The temperature dependence of electrical resistance for as-deposited and annealed gamma and delta polycrystalline silicide thin films exhibits a classic metallic behaviour (different from that of polycrystalline Ni thin films). The very different R(T) behaviour of as-deposited and annealed polycrystalline zeta-phase is interpreted taking into account the weak localization contribution.

Conductance measurements in thick-film sintered SnO2 samples with Au electrodes show that, besides temperature and the composition of the surrounding atmosphere, the applied de voltage also influences the conductance, both in steady-state and in transient conditions. This is interpreted in terms of the accumulation of a negative space charge (electrons trapped on levels distributed over an energy range in the forbidden gap) at the cathode interface.

A series of cuprates of type Bi-0212 has been prepared. The crystal structure was refined from neutron powder diffraction data at room temperature and 10 K as tetragonal (space group I4/mmm). Mossbauer and susceptibility measurements of the Fe substituted material indicate the development of a spin glass-like behavior at low temperatures. Superconductivity with T-c around 30 K is observed in the system Bi0.5La1.5-xYxSrCu2Oy. An increase of the hole-carrier concentration is obtained by suitable incorporation of Bi, Pb, La and Sr on the nine coordinated 4e positions with an improvement of T-c to 75 K.

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.