National Institute Of Materials Physics - Romania

A new model for the medium-range order in amorphous hydrogenated silicon is proposed that qualitatively explains a variety of experimental data. The model consists of a packing of continuous random network domains separated by distorted interfaces accumulating dangling and hydrogenated bands and hydrogen clusters.

The intensity dependent complex character of the photodarkening of virgin As2S3 samples exposed to continuous wave (CW) and pulsed laser radiation has been measured. Excitation in the CW and pulsed (picosecond) regime for the same irradiation doses leads to significant differences in the photodarkening curves. It is shown that the kinetics of the photodarkening can be decomposed into several processes related to structural transformations in As2S3: defect elimination and stabilization, polymerization of the structural entities, decomposition followed by arsenic clustering and arsenic oxidation.

Several cuprates of type Pb-0212 are prepared. The crystal structure is refined from neutron powder diffraction and X-ray diffraction data as tetragonal (space group 14/mmm). interestingly, Pb is simultaneously present in two atomic positions with CN 8 and 9 and considered as Pb4+ and Pb2+ Superconductivity with T-c between 30 and 75 K is observed in the system Pb0.5La1.5-xSr1+xCu2Oy. However, in as-prepared powdered materials the superconducting volume fraction remains low due to the formation of grain boundaries acting as weak links, whereas a significant improvement is obtained in bulk materials by use of flux. Mossbauer and susceptibility measurements of Fe substituted materials indicate the development of a spin glass like behaviour at low temperatures.

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.