• K P Gopinathan

      Articles written in Bulletin of Materials Science

    • Positron annihilation studies of rare-earth mixed valence compounds

      K P Gopinathan C S Sundar B Viswanathan A Bharathi

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      Positron lifetime has been measured in SmS under external pressures from 0 to ∼ 16 kbar, and in (Sm0·8Gd0·2)S at temperatures from 10 K to 300 K. An abrupt drop in the lifetime was observed at the valence transition at a pressure of 6·5 kbar in SmS and at a temperature of ∼ 120 K in (Sm0·8Gd0·2)S. Measurement of the Doppler broadening of the annihilation photons was made in (Sm0·8 Gd0·2)S as a function of temperature. The line shape parameter of the Doppler broadened spectrum also showed an abrupt change at the transition. Angular correlation of annihilation photons was measured in the semiconducting phase of SmS and in the metallic phase of (Sm0·8Tb0·2)S. The correlation profile in the mixed valence (metallic) phase was broader than that in the semi-conducting phase. These results have been explained on the basis of the electronic structure of the semiconducting and the metallic phases of these materials.

    • 181Ta TDPAC studies in proton implanted Nb

      R Govindaraj K P Gopinathan

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      Time differential perturbed angular correlation (TDPAC) measurement performed on the 482 keV level of181Ta probe nuclei inp-implantedNbHf sample shows that hydrogen decorated vacancy complexes are trapped by 6% of probe nuclei experiencing a Lorentzian distribution of quadrupole frequencies with a width of 32 MHz having a mean value of 545 ± 10 MHz with an asymmetry parameter 0·32. Measurements at RT following isochronal annealing treatment of the sample shows that the onset of detrapping of H-V complexes from probe nuclei occurs at 373 K and gets completed at 573 K. Hydrogen atoms stabilized vacancy clusters are found to be trapped by probe nuclei. The role played by other gaseous impurities in the sample in stabilizing the lattice defects bound to probe nuclei is discussed, to have a complete understanding of the recovery of defects in the sample.

    • 57Fe Mössbauer and positron lifetime studies of Al-Cu-Fe quasicrystals

      C M Chittaranjan T N Sairam K P Gopinathan C S Sundar

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      Al61·4Cu22·7Fe15·9, Al63·2Cu21·8Fe15 and Al71Cu14Fe15 have been studied by Mössbauer, positron annihilation and X-ray diffraction techniques. The positron lifetime, measured as a function of temperature, in Al61·4Cu22·7Fe15·9 shows a two-step increase. This is found to be due to the dynamics of phasons associated with Al and Cu, having activation energies of 0·43 eV and 0·78 eV respectively. In the case of Al71Cu14Fe15, a phase transformation from the quasicrystalline-crystalline phase at ≈ 550°C is observed.

    • Cold-worked and hydrogen chargedNbHf as studied by TDPAC and positron lifetime measurements

      R Govindaraj R Rajaraman K P Gopinathan B Viswanathan

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      Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.

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