In and Cd as defect traps in titanium dioxide

We present a study of TiO$_2$ single crystals from the point of view of the dopant atom that simultaneously behaves as the probing element. We used gamma-gamma time dependent perturbed angular correlations working with selected tracer elements ($^{111}$In/ $^{111}$Cd, $^{111m}$Cd/ $^{111}$Cd) together to investigate the different behavior of Cd and In dopants, particularly their interaction with point defects in the TiO$_2$ lattice. Results show that the hyperfine interactions observed at $^{111}$Cd from $^{111}$In or $^{111m}$Cd decay are quite different. $^{111}$In/ $^{111}$Cd results show a single site fraction characterized by a quadrupole frequency with asymmetry parameter similar to those observed for the same probe nuclei in bulk TiO$_2$ oxides. Results for $^{111m}$Cd/ $^{111}$Cd reveal two site fractions, one characterized by the same hyperfine parameters to those measured in bulk TiO$_2$ and another fraction characterized by a quadrupole frequency and asymmetry parameters with higher values, as observed in thin TiO$_2$ films and correlated with point defects. The results are discussed emphasizing the differences for Cd and In as defect traps on TiO$_2$.