PAC study of the static and dynamic aspects of metal atom inside C60 cage

30 keV 111mCd and 50 keV 199mHg beams from ISOLDE were used to implant on preformed target of C60 with the thickness of 1 mg/cm2. Endofullerene compounds, viz.,111mCd@C60 and 199mHg@C60 formed during implantation were separated by filtration through micropore filter paper followed by solvent extraction. Dried samples of the endofullerene compounds were counted for the coincidence for the 151-245 keV cascade of 111mCd and 374-158 keV cascade for 199mHg for the TDPAC measurement on a six LaBr3 detector system coupled with digital electronics. The results indicate a single site occupied by the Cd atom in the fullerene cage with fast relaxation component which means that the Cd atom does move inside the cage at room temperature. The quadrupole interaction frequency and asymmetry parameter of the Cd atom occupying the site in C60 are WQ=8.14(42) Mrad/s and eta= 0.42(9) respectively with a site population of 28%. Fast relaxation constant is 0.003 ns-1 with a site contribution of 72%. On the other hand, Hg atom has two static sites of quadrupole frequencies, WQ1=281.60(16.9) Mrad/s and WQ2=202.30(22.7) Mrad/s. Respective asymmetry parameters are eta1=0.1(1) and eta2=0.2(1). The static components have a contribution of 40% with the rest 60% coming from the fast relaxing component.

Study of surface-bulk mass transport and phase transformation in nano TiO2 using hyperfine interaction technique

Phase transition from anatase to rutile for the 70nm TiO2 crystallite has been investigated by annealing at different temperatures followed by TDPAC measurement of these TiO2 crystallites adsorbed with 181Hf tracer. The width of the peaks in XRD spectra for TiO2 crystallites annealed at different temperatures indicates the growth of the crystallites with temperature. The samples annealed upto 823K for 4hrs showed no phase transition, except the growth of the crystallites. However, it showed phase transition at the same temperature (823K), when annealed for longer duration. Annealing for shorter duration at 1123K showed phase transition. 181Hf tracer adsorbed on 70 nm anatase TiO2 is found to be in different polymorphs (anatase & rutile) formed during annealing process. Surface to bulk mass-transfer is found to play a significant role in the phase transition process.

The nuclear quadrupole interaction at 111Cd and 181Ta sites in anatase and rutile TiO2: A TDPAC study

The nuclear quadrupole interaction of the I=5/2 state of the nuclear probes 111Cd and 181Ta in the anatase and rutile polymorphs of bulk TiO2 was studied using the time differential perturbed angular correlation (TDPAC). The fast-slow coincidence setup is based on the CAMAC electronics. For anatase, the asymmetry of the electric field gradient was eta=0.22(1) and a quadrupole interaction frequency: 44.01(3) Mrad/s was obtained for 181Ta. For rutile, the respective values are eta=0.56(1) and quadrupole frequency=130.07(9) Mrad/s. The values for rutile match closely with the literature values. In case of the 111Cd probe produced from the beta decay of 111Ag, the quadrupole interaction frequency and the asymmetry parameter for anatase was negligible. This indicates an unperturbed angular correlation in anatase. On the other hand for rutile, the quadrupole frequency is 61.74(2) Mrad/s and the asymmetry is 0.23(1) for 111Cd probe. The results have been interpreted in terms of the surrounding atom positions in the lattice and the charge state of the probe nucleus.