National Institute Of Materials Physics - Romania

A resistance-heated furnace with a good control of temperature gradients is used for the Czochralski growth of precipitate and colour free Bi4Ge3O12 (BGO) crystals up to 30 mm in diameter and 80 mm in length from a 45 mm diameter crucible. The quality of the grown crystals is discussed in connection with the growth conditions. Spectrometric tests made on scintillators obtained from BGO crystals are presented. A Monte Carlo program is used in order to compute the full energy peak efficiency of the detector for different energies and geometries. The computed values are compared to the experimental results.

The location of the Fe3+ ions in the iron-containing scheelite structures of Bi3FeMO2O12 and Bi2Fe2Mo2O12 samples (which have been prepared under various temperature treatments) has been investigated by X-ray diffraction. Mossbauer and electron spin resonance (ESR) spectroscopies. The alteration of the crystalline structure and of the Fe3+ ion location have been followed after static reduction in propylene and reoxidation. The catalytic activity of the samples in the reaction of propylene oxidation to acrolein has been measured. The structural data have indicated for the Bi3FeMo2O12 samples tetragonal or monoclinic scheelite-type structure for the low (450-degrees-C) and high (560-degrees-C) temperature preparations, respectively, with the Fe3+ ions located in tetrahedral (4a) sites; this position is not affected by the reduction. A quantitative analysis of the structural data on Bi2Fe2Mo2O12 preparations led to the important conclusion that such a compositional phase exists only as a metastable, low-temperature one; at higher temperatures. Fe3+ ions in excess of Bi3FeMo2O12 composition are segregated on octahedral interstitial, ferric molybdate and/or alpha-Fe2O3 sites. Thus, this conclusion sheds light on the debated question of these phases as well defined compounds. Correlations between structure and catalytic activity data are discussed, taking into account the actual structures found in this study.

It is shown that X-ray powder diffraction patterns of mordenites can be used to reveal, by 2D Fourier mapping, structural changes due to chemical or thermal effects. During dealumination, a preferential localization of the structural defects at the tetrahedral groups of type 1 was observed. For different degrees of dealumination and cation contents, the localization of cations and/or water molecules inside the mordenite channels can be seen.

The maximum observed in the magnetic field dependence of the transport critical-current density of high-T(c) superconducting ceramics was analyzed. Transport critical current and magnetization measurements performed on Bi(Pb)-Sr-Ca-Cu-O bulk sintered samples allowed us to conclude that such a maximum may result from the influence of demagnetization effects at low applied field values and an increase of the intergrannular pinning at higher fields.

The influence of ammonium peroxodisulphate on the functional state of human hemoglobin was studied. The investigation techniques included: laser photolysis, spectrophotometry and pH measurements. Two different actions of the peroxodisulphate ions upon hemoglobin seem to occur; a) a proton consuming process and an increase of the 4th CO-binding rate constant, l'4, but not yet oxidation of the heme iron (when S2O8(2-) and heme concentrations were of the same order), and b) a marked oxidation effect (when S2O8(2-) was added in excess).

The relaxation of the darkening induced by nanosecond laser pulses in amorphous chalcogenide thin films (AsSe and As2Se3) was modelled on the basis of a non-archimedean (ultrametric) approach. While for AsSe only a single time scale of evolution was evidenced (tentatively related to the simple, molecular structure of the glass) in As2Se3 three time scales were evidenced and discussed in the frame of a complex hierarchical structure of the glass with layer fragments, clusters of layers (amorphites) and, possibly, bigger domains.

The results of magnetic measurements performed on ZrCo5.1-xFex alloys in the temperature range 4.2-900 K and fields up to 7 T are reported. The presence of solid solutions is evidenced in the composition range x < 0.9. The Curie temperature and the saturation magnetization increase nearly linearly when cobalt is substituted by iron. In addition, the anisotropy field decreases and shows an unusual temperature dependence, with a maximum at T congruent-to 300 K.

A Hendricks-Teller analysis of the X-ray diffraction line shifts in Bi-Sr-Ca-Cu-O phases allows an estimate of the stacking fault probabilities (missing or extra Cu-O layers). The probability values suggest some conclusions about the mutual relationships of the 2201, 2212 and 2223 phases and the kinetics of their formation.

The GdCo4-xNi(x)B alloys, for x less-than-or-equal-to 3 and GdCo4-xFe(x)B alloys for x less-than-or-equal-to 1.5 crystallize in a hexagonal structure of CeCo4B-type. The magnetic measurements performed in the temperature range 4.2-900 K show that GdCo4-xM(x)B compounds (M = Fe or Ni) are ferrimagnetically ordered. The mean cobalt moments increase when cobalt is substituted by iron and decrease when it is replaced by nickel. Above the Curie temperatures, the reciprocal susceptibilities for GdCo4-xNi(x)B alloys show a nonlinear temperature dependence. The exchange fields acting on cobalt atoms were evaluated. The variations of the cobalt moments are proportional to those of the exchange fields.

Using a simple Foldy-Lax model for the refractive index of photons in a nuclear medium and actual empirical gamma-N forward Compton scattering amplitudes we show that the "coherence condition" for the emission of gamma-Cerenkov radiation is fulfilled at incident energies higher than 16 GeV/nucleon in the gamma-spectral band 140 less-than-or-equal-to omega less-than-or-equal-to 160 MeV, still well below the DELTA resonance region. The predictions for gamma-cross sections are between 62-mu-b and 248-mu-b for projectile nuclei with 16 less-than-or-equal-to A less-than-or-equal-to 32. Also the complete linear polarization of the coherent gamma-Cerenkov radiation is pointed out.