J. Kesten

The EFG of the In-O-Bond In2O3, AgO and Ag2O

PAC measurements on111In in In2O3, AgO and Ag2O were performed to study the influence of different oxygen coordinations to the EFG.111In was implanted at 500keV; in addition, AgO and Ag2O were irradiated in the cyclotron to produce111In via the (α, 2n) reaction. Using the point charge model and the known lattice structure of the oxides, the antishielding factor was calculated; it shows a strong dependence on the In-O bondlength.

Structural phase transition of Rh2O3 studied via PAC

The structural phase transition from α-Rh2O3 to β-Rh2O3 has been studied via the PAC method on dilute111Cd impurity atoms implanted into α-Rh2O3. The measured antishielding factor β=112 (2) in the α-phase is similar to that found for111Cd in other oxides with corundum structure (Al2O3. Cr2O3, Fe2O3). Oxygen with different crystallographic matrices show a pronounced correlation of the electric field gradient with the NN oxygen coordination.

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive111In+ ions were implanted into an α-Al2O3 single crystal. The hyperfine parameters of111Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientVzz=1.04(17)·1022V/m2 was obtained from the quadrupole coupling constant. This result is compared with the efg values of27Al in α-alumina and111Cd in α-Fe2O3, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to111Cd atoms in a strongly distorted Al2O3-lattice.

Oxidation of indium implanted aluminium studied by the Rutherford backscattering and perturbed angular correlation methods

Aluminium foils oversaturated with 5.1015/cm2 implanted115In+ ions were oxidized in 200 mbar oxygen or vacuum annealed at 370–870 K. The samples were analyzed by the Rutherford backscattering (RBS) and perturbed angular correlation (PAC) techniques, using some 1011/cm2 implanted radioactive111In tracers. Furthermore, the oxygen surface profiles were also scanned with high resolution by using the nuclear resonance technique (NRA). The formation of passivating Al2O3 surface layers, preventing deeper oxygen diffusion and the indium diffusion into these oxidized surface layers and into the bulk, were studied. Several quadrupole interaction frequencies previously attributed to strained cubic indium precipitates and indium (-vacancy) clusters were observed. When the samples were oxidized above 750 K, the formation of In-O complexes and of substitutional111In in Al2O3 was observed.

Hyperfine interactions of111Cd impurities in Cr2O3

The electric and (antiferro-)magnetic hyperfine interaction of111Cd in α-Cr2O3 after111In implantation was studied via PAC. Two fractions with axially symmetric electric field gradients were observed having antishielding factors of Vzz/Vzzpc=79(12) and 106(16), respectively. From the line broadening of the Fourier components of the first fraction its supertransferred magnetic field was estimated as Bloc (20 K)≤0.41 T, about two orders of magnitude smaller than for substitutional111Cd in NiO and CoO.

Magnetic hyperfine interaction after77Br implantation into iron

Radioactive77Br ions were implanted at 60 keV and 270 keV into polycrystalline iron foils. The magnetic hyperfine interaction of the daughter nucleus77Se in the 250 keV isomeric level was investigated using the perturbed angular correlation technique and fast BaF2 detectors. The Larmor frequency at room temperature was determined as ωL=1272(2) MHz, leading to a substitutional hyperfine field ofBhf(SeFe)=62(6) T. The substitutional fraction rises from 21% at room temperature to about 100% at 790 K annealing temperature. No other well-defined magnetic or electric hyperfine component attributable to impurity-defect complexes was identified. The diffusion of Br atoms in the surface region during the annealing process was studied via Rutherford backscattering with 1.0 MeV α-particles.