Part 1: A particular EFG temperature dependence for $^{181}\textbf{(TiO}_2)$: An electron-gamma TDPAC study.

The hyperfine parameters of the $^{181}$Ta trace impurities in the rutile structure of the TiO$_2$ single crystals (denoted 181Ta(TiO2)) are studied mainly using the time differential e-γ perturbed angular correlation (e-γ TDPAC) technique by the ion-implanted $^{181}$Hf tracers that decays to 181Ta. The experiments are performed in the air, with a temperature range of 50 − 427 𝐾. It is found that with significant annealing (5hr 15mins at a temperature of 873.15 𝐾) of the $^{181}$Ta(TiO$_2$) sample, the $^{181}$Ta trace impurities are mainly found in the main interstitial Ti site. This allows for the precise measurement of the largest electric field gradient (𝑉𝑧𝑧), asymmetry parameter (𝜂), and orientation (𝜃 & 𝜙) of the interstitial $^{181}$Ta trace impurities surrounded by the octahedral-like TiO2 host matrix surroundings. The 𝑅(𝑡) signal and the experimental parameters resulting from the e-γ TDPAC spectroscopy agree with that of the γ-γ TDPAC spectroscopy (in both our experiment and literature) at room temperature. However, we detected a non-linear increase of 𝑉𝑧𝑧 (with a positive curvature) at the low-temperature regime (50 − 293 𝐾), as opposed to the linear increase at the high- temperature regime (600-1200 K) as suggested by the literature. We also suspected that at a specific temperature of around 500 𝐾, there might be a 𝑉𝑧𝑧 maxima. Though precise ab-initio methods modelled the linear increase at the high-temperature regime, such techniques are lacking for the low-temperature regime. Hence, our next step would be doing Density Functional Theory (DFT) calculations on the Ta(TiO$_2$) over a broad temperature range of 0 − 1200 𝐾 to obtain more profound insight into the particular temperature dependence of our experimentally measured 𝑉𝑧𝑧 in the theoretical part of this thesis (Part 2).