Electric field gradients at 181Ta sites in HfOx

In the present work we report preliminary results about the possibility to study properties of the order-disorder transition in HfOx solid solutions via the determination of the electric field gradient (EFG) at 181Ta radioactive probes. Oxygen solution into the metal was achieved by arc melting stoichiometric amounts of metallic Hf and HfO2 under argon atmosphere. Samples of HfOx with x=0.1 and 0.2 were prepared. Two types of samples were used for the perturbed angular correlation (PAC) experiments by doping alternatively with 181Ta, by neutron irradiation, the metallic Hf or the hafnium oxide. The PAC results on both samples were identical, with disappearing hyperfine signals of the metal and the oxide, showing a complete diffusion of the probes independent of the way of doping. The PAC signal of the HfOx solid solution consisted in a wide distribution of EFGs due to the oxygen disorder. This scheme held even after long thermal treatments at high temperature (several days at 1273K). Annealing treatments at moderate temperature (600K) were also made. In these cases the samples were cooling at a very low rate. These results, together with those obtained by measuring samples below and above the order temperature are reported.