The Electrical Field Gradient at $^{181}Ta$ in $ZrSiO_{4}$ and $HfSiO_{4}$

We have used perturbed angular correlation spectroscopy (PAC) to measure the electric field gradient (\textit{EFG}) at the \textit{Zr}-site in zircon (\textit{ZrSiO}$_{4})$ between room temperature and \textit{1100\r{ }C} using $^{181}$\textit{Ta} probe nuclei. The \textit{EFG} is axially symmetric and the quadrupole interaction frequency \textit{$\nu $}$_{Q}$ decreases linearly with increasing temperature. While for some zircon specimens the slope of the \textit{$\nu $}$_{Q}$\textit{ vs. T} increases above \textit{900\r{ }C}, for others this change in slope is not observed. This change in the thermal behavior \textit{$\nu $}$_{Q}$ is consistent with a displacive structural transition reported in the literature.\footnote{Z. Mursic, T. Vogt, and F. Frey, Acta Cryst. \textbf{B48} (1992) 584.} In order to learn more about this structure we have begun performing PAC experiments on isostructural hafnon (\textit{HfSiO}$_{4})$, which was synthesized in our laboratory. PAC spectra of hafnon are very similar to those of zircon but show a small second-site interaction due to residual \textit{HfO}$_{2}$ from the sample preparation. The temperature dependence of the quadrupole interaction frequency will be discussed in context of a displacive phase transition in these materials.