  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.
