TDPAC-Studies of Electric Field Gradients in Amorphous Metallic Systems

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TDPAC-STUDIES OF ELECTRIC FIELD GRADIENTS IN AMORPHOUS METALLIC SYSTEMS P. HEUBES, D. KORN, G. SCHATZ and G. ZIBOLD Fakultdt fUr Physik, Universitdt Konstanz, D-7750 Konstanz, Fed. Rep. Germany ABSTRACT The time differential perturbed y-y angular correlation technique (TDPAC) is applied to the amorphous metallic systems Ga, Bi, In 5 0 Au 5 0 and In oAg o. TheljIectric field gradient Cd nuclei shows a broad tensor proDed by probability distribution with a relative width of 0.4 - 0.5 for all systems, as suggested by a continuous random structural model. INTRODUCTION Outstanding features of amorphous metals are their local atomic arrangement, lacking long-range periodicity, and their electronic structure. In recent years several structural models have been proposed which are roughly characterized in terms of "microcrystalline" and "continuous random" models [1,2]. In case of amorphous metals and metallic alloys,the radial distribution analysis from diffraction studies favours the dense random packing hypothesis although an unambiguous determination by diffraction techniques is principally not possible [1]. Recently additional information on the local atomic arrangement has been derived from the electric field gradient (EFG) acting onto probe atoms [3,4]. This quantity depends on the symmetry of the charges surrounding the probes and therefore provides direct experimental information on the angular distribution of local atomic arrangements. The time differential perturbed angular correlation technique (TDPAC) complements other EFG sensitive methods (e.g. Mbssbauer spectroscopy), since one can work with a low number of probes (> 10 probe nuclei per sample). This special feature turns out to be important in the case of metallic systems since most of these samples are only available in the amorphous state by quench condensation onto cooled substrates keeping the deposited films below a critical thickness. Mbssbauer effect experiments, however, require an absorber thickness not below a certain thickness (typically 1 nm) in order to obtain a reasonable absorption effect. This restriction is not present in TDPAC investigations. Another feature related to the exclusive EFG sensitivity of the TDPAC technique is avoiding possible complications from other interactions (e.g. isomer shifts). Since samples with structural disorder usually exhibit a broad distribution of interaction frequencies, an unequivocal separation of different contributions is a very difficult task. The conclusions drawn from TDPAC results, however, are not affected by this ambiguity.

386

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300 200 Temperature ICK

1

Electrical resistivity of quench-condensed function of temperature.

In-Ag alloys as a

In the present work we have extended previous investigations of amorphous Ga [4] to Bi metal and In-Au and In-Ag metallic }Joys. For th 1TDPAC measurements the 172-247 keY y-y cascade o f C after In decay was used with the intermediate state I= 5/2 (T = 84 ns). In

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