Electric field gradients in Zr$_8$Ni$_{21}$ and Hf$_8$Ni$_{21}$ intermetallic compounds; Results from perturbed angular correlation measurements and first-principles density functional theory

Abstract: Numerous technological applications of Ni-based Zr and Hf intermetallic alloys promoted comprehensive studies in Zr$8$Ni${21}$ and Hf$8$Ni${21}$ by perturbed angular correlation (PAC) spectroscopy, which were not studied earlier until this report. The different phases produced in the samples have been identified by PAC and X-ray diffraction (XRD) measurements. Using $^{181}$Hf probe, two non-equivalent Zr/Hf sites have been observed in both Zr$8$Ni${21}$ and Hf$8$Ni${21}$ compounds. From present PAC measurements in Zr$8$Ni${21}$, a component due to the production of Zr$7$Ni${10}$ by eutectic reaction from the liquid metals is also observed. The phase Zr$7$Ni${10}$, however, is not found from the XRD measurement. In Zr$8$Ni${21}$, while the results do not change appreciably up to 973 K exhibit drastic changes at 1073 K. In Hf$8$Ni${21}$, similar results for the two non-equivalent sites have been found but site fractions are in reverse order. In this alloy, a different contaminating phase, possibly due to HfNi$3$, has been found from PAC measurements but is not found from XRD measurement. Density functional theory (DFT) based calculations of electric field gradient (EFG) and asymmetry parameter ($\eta$) at the sites of $^{181}$Ta probe nucleus allowed us to assign the observed EFG fractions to the various lattice sites in (Zr/Hf)$_8$Ni${21}$ compounds.