Thermal annealing effects in polycrystalline EuTiO$_{3}$ and Eu$_{2}$Ti$_{2}$O$_{7}$

Using time-differential perturbed angular correlation ofγrays, we investigated the electric-field gradient of polycrystalline EuTiO$_{3}$ and Eu$_{2}$Ti$_{2}$O$_{7}$, with $^{181}$Hf($^{181}$Ta) as a probe, following different thermal treatments. The measurements were performed at ISOLDE-CERN fol-lowing 80 keV implantation at the Bonn Radioisotope Separator. The experimental results indicated successful induction of different phases in the implantation recovery process at 1273 and 1373 K. These observations were combined with $ab initio$ calculations and X-ray diffraction measurements. A comparison of $ab initio$ calculated electric-field gradients with the measured values discriminates between different structures and defects and rules out many possible cases. The Ta probe at the Ti site in the Eu$_{2}$Ti$_{2}$O$_{7}$ phase is found to be the most probable case of site occupation after annealing at 1373 K, while annealing at 1273 K keeps EuTiO$_{3}$ in the vicinity of the Ta probe. A discussion of the hyperfine interactions that promote variation in the interaction strength at the $^{181}$ Ta site is presented.