The HfAl2 and ZrAl2 Laves phases studied by 181Ta and 111Cd perturbed angular correlation spectroscopy

Using perturbed angular correlation (PAC) spectroscopy and Hf-181/Ta-181 and In-111/Cd-111 radioactive probes, the electric field gradients (EFGs) in the ZrAl2 and HfAl2 Laves phases were measured in the temperature range 20-1100 K. In both matrices, Hf-181 was found to substitute at the unique Hf/Zr site. In the ZrAl2 compound, the In-111 probes were assigned to the two Al sites. The relative fractions of the corresponding EFGs indicate a strong preference of these impurities in favor of the 2(a) Al site over the 6(h) Al site. In the HfAl2 compound, the In-111 impurities substitute for Hf up to 370 K and reach the Al sites only above 420 K. In both compounds and for both radioactive probes, the temperature dependence of the quadrupole interaction frequencies was measured. It is interesting to note that, for Ta-181 in both compounds, they increase with temperature. (C) 2002 Elsevier Science B.V. All rights reserved.