Electric Field Gradient at ¹⁸¹Ta Nuclei in Hafnium Coordination Compounds

Abstract

<p>The quadrupole interaction of ¹⁸¹Ta with the electric field gradient (EFG) at Hf sites in the coordination compounds (NH₄)₃ HfF₇, K₃HfF₇ and Hf(acac)₄ has been measured by the method of differential perturbed angular correlations.</p> <p>The heptafluorates which possess identical crystal structures were found to produce vastly different EFG's at the sites of the ¹⁸¹Ta probe nuclei. Over the range from liquid nitrogen to room temperature, the EFG in (NH₄)₃HfF₇ was observed to change from a static to a dynamic nature due to the reorienting HfF₇⁻³ ion add the effects of the dynamic NH⁴⁺ ions. No comparable time-dependent effects were observed in K₃HfF₇ which required a two site static model for the Hf atoms at liquid nitrogen temperature and a one site model, with only a slight dynamic modification, at room temperature. The results were found to be in agreement with other NMR and X-ray studies of these compounds.</p> <p>The measurement of the EFG at the metal site in Hf(acac)₄ yielded a large asymmetry value that is discussed in terms of the crystal structure and type of ligand bonding involved in this octocoordinated complex. Comparison of the results for synthetic and irradiated samples of Hf(acac)₄ gave no indications of radiation produced damage to the crystal structure. Evidence for the existence of two sites with different asymmetry parameters as suggested by a previous study was not found.</p> <p>A detailed investigation of instrumental effects in perturbed angular correlation measurements was also carried out. This study included the measurement and correction of system nonlinearities and background anomalies in time spectra measurements.</p> <p>Finally, a precise measurement of the half-life of the 482 keV level in ¹⁸¹Ta was performed using the delayed coincidence method.</p>