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Intermetallic compounds Zr7Ni10 and Hf7Ni10 have been studied by perturbed angular correlation (PAC) spectroscopy considering the fact that Zr7Ni10 has application as hydrogen storage material in fuel cell. In stoichiometric Zr7Ni10, the phase Zr2Ni7 is found to be dominant (~38%) while a fraction of ~25% is found for the Zr7Ni10 phase at room temperature. In this compound, a phase due to Zr8Ni21 (~10%) is found from room temperature up to 773 K but, this is not found at 873 K and above. In the stoichiometric Hf7Ni10 sample, the phase due to Hf7Ni10 is found as a minor phase (~22%) at room temperature. In this system, no phase of Hf2Ni7 is observed but, a different phase due to HfNi3 is found to be dominant (~62%). It is found that the site fraction of Hf7Ni10 enhances with temperature at the expense of HfNi3 and this phase becomes predominant (~57%) at 673 K and above. The change of phase fractions of HfNi3 and Hf7Ni10 with temperature is found to be reversible. The phase components in Zr7Ni10 and Hf7Ni10 have been determined also from X-ray powder diffraction (XRD) and transmission electron microscopy/selected area electron diffraction (TEM/SAED) measurements. Ab-initio calculations using the all electron full potential (linearized) augmented plane wave [FP-(L)APW] method, within the framework of the density functional theory (DFT), have been performed to determine the electric field gradients at the 181Ta impurity sites and therefrom to assign the different components observed from PAC measurements. Present experimental and calculated results of EFG support the Cmca space group for both Zr7Ni10 and Hf7Ni10 compounds., Comment: arXiv admin note: text overlap with arXiv:1712.04258

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.