A Time Differential Perturbed Angular Correlation Study of Ultrapure Hafnium Metal

Prof. Dr. Tilman Butz

Hf metal crystallizes in the hcp strucure. By thermal neutron activation ¹⁸¹Hf (t_1/2 = 42 d) is produced which is a suitable isotope for Time Differential Perturbed Angular Correlation (TDPAC) of  -rays. Contrary to expectations, one does not find 100% of all probe atoms in an axially symmetric electric field gradient. Three further signals are observed with about 3% - 5% of the total signal intensity and asymmetry parameters of 0, about 0.5, and about 0.9 (see Fig.1). The samples contain only 110 ppm Zr, much too low for being responsible for the extra signals.

Figure 1: Cosine-transformed TDPAC spectrum of ¹⁸¹Hf(b^-)¹⁸¹Ta in ultrapure Hf metal.

Moreover, Zr and Hf are chemically very similar. Thus we have to look for another explanation. These extra signals are tentatively assigned to one, two, and three atoms on interstitial positions. One of the signals has axial symmetry and requires a first coordination shell with three-fold rotation axis. Incidentally, this signal resembles the signal observed in Zr on B-sites with double occupancy. Heterophase fluctuations have been proposed long time ago for the group Ivb metals based on Mößbauer and neutron diffraction data. We believe that the recoil during prompt de-excitation of ¹⁸¹Hf in a highly excited state after thermal neutron capture might lead to such metastable configurations because at about 100°C these extra signals are essentially lost. Heating and quenching experiments are under way to clarify the situation.