National Institute Of Materials Physics - Romania

Electron spin resonance and optical studies reveal the presence of both Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagnetic Cu2+ centers with different concentrations and production properties have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedron. The less abundant Cu2+(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, part of it being afterwards converted to Cu+ precursor centers. The presence of a neighboring-charge compensating anion vacancy and its departure during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) centers, respectively.

Anomalies occurring in irradiated polymers within the glass transition range, as reported by various authors are critically reviewed. A theoretical description for such processes, is developed within the free volume approximation. The discrepancies between the temperature at which such anomalies are noticed and the glass transition temperature have been ascribed to the fact that the volume of radiation-induced radicals is not equal with the segmental one. The agreement between experiment and theory is good. (C) 1997 Elsevier Science B.V.

Lanthanum stearate films were prepared by the conventional Langmuir-Blodgett method and by a dragging procedure. Multilayers have been obtained and studied by X-ray diffraction. The electron density distribution normal to the bilayer plane has been calculated. (C) 1997 Elsevier Science S.A.

The light induced modifications in thin films of Fe:PVA were studied by optical and Mossbauer spectroscopy. Under the UV exposure both the refractive index and the absorption coefficient are changing. The real part of the refractive index is measured using m-line Spectroscopy. As a function of UV exposure, the optical absorption as well as the Mossbauer spectra were measured. A correlation between the optical and the measured Fe2+/Fe3+ ratio is discussed. (C) 1997 Elsevier Science S.A.

The concept of NIEL (Non Ionising Energy Loss) is usually considered to correlate the effects of the displacement damage produced in the material bulk by different particles in different energy ranges. An evaluation of the NIEL is especially important for those semiconductors which are planned to be used in the extreme radiation environment of the new generation of colliders as LHC. In the present work, a calculation of the pion NIEL in silicon and GaAs, in the energy region up to 50 GeV is reported and discussed. The energy dependence of the NIEL is found to follow the resonant structures of the pion-nuclei interactions, where the strongest effect is due to the Delta(33) resonance. Above 1 GeV a slight monotonic decreasing behaviour of the NIEL has been found, both for Si and GaAs.

The Fe:PVA thin films were studied by optical and Mossbauer spectroscopy. Both the absorption and Mossbauer spectra are dependent on the PVA dilution and ferric chloride concentration. A strong correlation between the optical and Mossbauer results was found, indicating the major role of the iron electronic local levels in the optical phenomena. A relation between the various absorption probabilities and also indirect information about the Debye temperatures were obtained. (C) 1997 Elsevier Science S.A.

A magnetic susceptibility and Mossbauer spectroscopy study of the perovskite Bi2FeRhO6 was undertaken. A freezing start temperature around 270 K was evidenced by both methods. Unlike homogeneous magnetic systems the freezing occurs by involving continuously more and more Fe spins with decreasing temperature over the whole interval down to 4.2 K. At low temperature some evidence for long range antiferromagnetic interactions was found. The Mossbauer data allowed a satisfactory description of the peculiar temperature dependence of the magnetic susceptibility as a superposition of a slain glass antiferromagnetic component and a paramagnetic one. The gradual spin freezing and the absence of percolation derived from the strong influence of the short range interactions, their strengths reflecting the random distribution of the Fe and Rh ions over the B sites of the ABO(3) perovskite. (C) 1997 Elsevier Science S.A.

A Mossbauer spectroscopy study of the intermetallics RE6Fe14-xMx where RE=Nd or Pr, M=Ga or Al and x=1 or 2 was undertaken. A comparative analysis of the Mossbauer spectra showed that the second Al or Ga atom substitutes preferentially for the Fe atoms located at the 161 sites. The reduction of the hyperfine fields in Nd6Fe12M2, as compared with Nd6Fe13M, mainly at the 16k, 161, and 161(2) and the corresponding decrease of the average Fe moment gives rise to a total magnetic moment of 41 mu(B) per formula unit in excellent agreement with previous saturation magnetisation measurements.

Fresh silica gels prepared by the sol-gel route with acid catalysis display typical alterations of the X-ray diffraction (XRD) pattern, in comparison with vitreous (v) silica. A positive shift of the first peak at k(1) is noticed, which also shows excess broadening. The changes can be accounted for by the alteration of the intermediate-range order (IRO) in gels; in comparison with v-SiO2 Thermal annealing causes a gradual decrease in the shift and broadening of the first peak, suggesting that the evolution of the vitrification can be followed on the basis of XRD data. Two types of IRO alteration were modelled, which account quantitatively for the experimental observations using a phe nomenological disorder parameter b. The evolution of the b values with annealing temperature reveals well-defined stages in the vitrification process.

Metastable phases can appear in metallic alloys if their surface response function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. in this case, rapid solidification procedures can result in their formation. On the other hand, metastable alloys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decomposition. These microstructures depend on the thermal history of the sample (e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag1-xGex (x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase is the Hume-Rothery type. Besides, an alpha-Ag-Ge solid solution was present. Amounts of non-reacted Ge were also found, with a lattice constant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatures of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks appeared with relatively high heat release (46.6 and 48.8 J g(-1), respectively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidification process and are expected to affect their stability. (C) 1997 Elsevier Science S.A.