  Time-differential perturbed angular correlation (TDPAC) measurements have
been carried out in stoichiometric ZrNi$_3$ and HfNi$_3$ intermetallic
compounds using $^{181}$Ta probe in the temperature range 77-1073 K considering
the immense technological applications of Zr-Ni and Hf-Ni intermetallic
compounds. In ZrNi$_3$, four components due to the production of Zr$_2$Ni$_7$,
Zr$_8$Ni$_{21}$, Zr$_7$Ni$_{10}$ and ZrNi$_3$ have been found at room
temperature. The HfNi$_3$ sample produces five electric quadrupole interaction
frequencies at room temperature. The phase HfNi$_3$ is strongly produced in
stoichiometric sample of HfNi$_3$ where two non-equivalent Hf sites are found
to be present. Besides this phase, two other phases due to Hf$_2$Ni$_7$ and
Hf$_8$Ni$_{21}$ have been found but, we do not observe any phase due to
Hf$_7$Ni$_{10}$. X-ray diffraction, TEM/energy dispersive X-ray spectroscopy
(EDX) and TEM-selected area electron diffraction (SAED) measurements were used
to further characterize the investigated materials and it was found that these
results agree with the TDPAC results. In order to confirm findings from TDPAC
measurements, density functional theory (DFT) based calculations of electric
field gradients (EFG) and asymmetry parameters at the sites of $^{181}$Ta probe
nucleus were performed. Our calculated results are found to be in excellent
agreement with the experimental results.
