Orientation of an oxygen-induced non-axially symmetric electric field gradient at Ta in Nb

Abstract

The interaction of dilute Hf in Nb with interstitially dissolved oxygen has been investigated via the γ-γ-TDPAC-technique applied to the 482 keV state of181Ta. After melting the parent radioactivity181Hf with Nb the trapping of oxygen at the probe atoms leads to a quadrupole interaction frequency ν Q =268(1) MHz with an asymmetry parameter of η=0.68(1). The temperature dependence of this frequency between 17 K and 293 K was found to be very weak whereas η varies considerably. In a single crystal experiment the orientation of the three principal axes of the electric field gradient tensor leading to ν Q has been determined. TheV zz -axis points into a 〈110〉, theV yy -axis into a 〈100〉 and theV x -axis into a 〈110〉 direction. The results can be explained in the point charge model by assuming that one oxygen atom is trapped on the second nearest neighbour octahedral position next to the substitutional atom and the covalent bonds between the daughter Ta and the oxygen lead to a localization of charge near the probe atom.