A. R. López-García’s research while affiliated with National University of La Plata and other places

The crystallization of Hf100−xCux (x=33,44,50,59) amorphous alloys was studied by the TDPAC technique. The different stages in the transformation towards equilibrium were investigated through the evolution of the quadrupole perturbation after thermal annealings. The crystallization kinetics of Hf67Cu33 and Hf56Cu44 was analyzed using the Johnson-Mehl-Avrami equation. General trends in the crystallization behavior are discussed.

The thermal behavior of the hyperfine interaction in K2HfF6 was investigated by means of the time-differential perturbed-angular-correlation technique between 293 and 750 K. The quadrupole parameters associated with the monoclinic, orthorhombic, cubic, and trigonal phases have been determined. An activation energy Ea=86±16 kJ/mol could be calculated for the monoclinic-to-orthorhombic phase transition taking place between 384 and 466 K. An analogy with the hyperfine interaction of the isomorphous compound K2ZrF6 could be established.

The compound CdCl2 was investigated by TDPAC searching for aftereffects at111Cd probes. In opposition to previous results, we do not find such effects. This confirms that, as previously suggested, the probe must be a non-isovalent impurity in the compound, so that the atomic disturbances following electron-capture persist long enough to produce detectable perturbations in the angular correlation.

The hyperfine interactions at In and Sn sites of In2O3Sn (ITO) were measured through time-differential perturbed angular correlations and Mssbauer spectroscopy, respectively. Polycrystalline samples prepared by co-precipitation with nominal 0.025, 1 and 5 at.% Sn were studied. They all showed the cubic bixbyite structure characteristic of In2O3 after annealings at 200 C. The quadrupole interaction at In sites appears nearly independent of Sn concentration being the main result of the presence of Sn in the lattice, the gradual disappearence of the dynamic perturbation caused by after-effects. The Mssbauer data demonstrate that Sn ions are in a 4+ state with Q=0.606 mm/s and=0.226 mm/s relative to SnO3Ca at room temperature.

The results show that it is possible, by controlling the oxide particle size and the average distance between them, to modify the internal oxidation kinetics, indicating that oxygen diffusion can be altered. There is indirect evidence that inhomogeneous elastic fields are responsible for the rapid access channels for diffusion when the oxide complex concentration is high enough.

The Mössbauer parameters of anhydrous monoclinic tin/II/ phosphate, Sn3/PO4/2, are reported and briefly discussed. The results agree with the interactions expected from the crystalline structure but differ from previously reported data.

The Mssbauer parameters of anhydrous monoclinic tin/II/ phosphate, Sn3/PO4/2, are reported and briefly discussed. The results agree with the interactions expected from the crystalline structure but differ from previously reported data.

The general problem of anomalous diffusion is discussed in connection with the way in which the TDPAC (time differential perturbed angular correlation) technique may help determine the relative fraction of substitutional or interstitial positions of the diffusing atoms. The TDPAC technique referred to the Ag-In system is outlined. The present measurements give a fraction of 0.89+or-0.21 of Ag solutes in In substitutional positions. This suggests a dissociative mechanism for Ag diffusion in In. The value of 0.6+or-0.5 is obtained for the electric field gradient asymmetry parameter eta 2 of the assumed interstitial positions. This value shows a low symmetry for the interstitial sites, indicating that these positions might be split interstitials of Ag solutes in In.