A microscopic study of the interaction between aliovalent dopants and native defects in group IV oxides : indium and cadmium in ceria and zirconia

In order to understand better the defect structure and dynamics associated with lower valent dopants complexed with native defects in group IV oxides, In/Cd perturbed angular correlation spectroscopy was performed in ceria and zirconia. Examining the orientation symmetry axis of defects in ceria single crystals at low temperature has allowed the identification of a cadmium with a bound near-neighbor oxygen-vacancy complex as well as a complex involving a cadmium with two opposing, near-neighbor oxygen vacancies. The orientation of the symmetry axis of a third complex is reported; however, this information is not sufficient to identify it. Complementing these low temperature studies, the dynamics of the cadmium/oxygen-vacancy interaction in zirconia at high temperatures was studied. The motion of the oxygen vacancy at high temperatures results in a damping of the PAC signal. This damping is not well characterized by the heuristic Marshall-Meares PAC fitting function, and a model is proposed to fit the data in terms of three physical parameters associated with the vacancy's motion. These parameters are the rate at which a bound oxygen vacancy hops among equivalent sites about the probe, the rate at which a bound vacancy detraps, and the rate at which a vacancy is trapped by cadmium. Fits of individual spectra using this model give respective activation energies of 0.3-0.6 eV, 0.9-1.6 eV, and 0.4-0.6 eV. The uncertainty in these energies can most likely be reduced by fitting spectra from multiple temperatures simultaneously. Despite the large uncertainty in the fitted energies, the values are physically reasonable and indicate that the model adequately describes the motion of the oxygen vacancy about cadmium

Site occupation of indium and jump frequencies of cadmium in FeGa3

Perturbed angular correlation (PAC) measurements using the In-111 probe were carried out on FeGa 3 as part of a broader investigation of indium site occupation and cadmium diffusion in intermetallic compounds. One PAC signal was observed with hyperfine parameters ω 1 = 513.8(1) Mrad/s and η = 0.939(2) at room temperature. By comparison with quadrupole frequencies observed in PAC measurements on isostructural RuIn 3 , it was determined that indium occupies only the 8j site in the FeGa 3 structure, denoted Ga(2) below because two out of the three Ga sites have this point symmetry. PAC spectra at elevated temperature exhibited damping characteristic of electric field gradients (EFGs) that fluctuate as Cd probes jump among Ga(2) sites within the lifetime of the excited PAC level. A stochastic model for the EFG fluctuations based on four conceivable, single-step jump-pathways connecting one Ga(2) site to neighboring Ga(2) sites was developed and used to fit PAC spectra. The four pathways lead to two observable EFG reorientation rates, and these reorientation rates were found to be strongly dependent on EFG orientation. Calculations using density functional theory were used to reduce the number of unknowns in the model with respect to EFG orientation. This made it possible to determine with reasonable precision the total jump rate of Cd among Ga(2) sites that correspond to a change in mirror plane orientation of site-symmetry. This total jump rate was found to be thermally activated with an activation enthalpy of 1.8 ±0.1 eV

Comparison of XYZ Model Fitting Functions for 111 Cd

The XYZ model describes the interaction between nuclear probes and an electric field gradient that fluctuates among three orthogonal directions. The model presents a means to calculate the perturbation function that represents spectra obtained using perturbed angular correlation spectroscopy. Three analytic approximations of the perturbation function have been developed previously, and they are evaluated in the present paper in the context of Cd jumping among In-lattice sites in In3La