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

Search for Nucleation of Phase Embryos in Binary Alloys by Impurity Atoms

A search was made to detect the possibility that individual impurity atoms in two-phase, binary alloys nucleate embryonic crystals. The alloy system studied was Ni-Al and the impurities were ln probe atoms. Local surroundings of the probes characteristic of the crystal phases were detected through quadrupole interactions with Cd daughter nuclei using the method of perturbed angular correlation of gamma rays. Analysis of site fractions of the probes led to two alternative interpretations that could not be distinguished on the basis of the present measurements: 1.) ln probes in the two-phase domain between Ni Al and NiAl segregate to Ni Al with a segregation energy of about 0.10 eV; or 2.) ln probes nucleate embryonic crystals of Ni Al , with the driving force for nucleation being an attraction between In atoms and Ni vacancies that is known to be strong in NiAl

Search for Nucleation of Phase Embryos in Binary Alloys by Impurity Atoms

A search was made to detect the possibility that individual impurity atoms in two-phase, binary alloys nucleate embryonic crystals. The alloy system studied was Ni-Al and the impurities were ln probe atoms. Local surroundings of the probes characteristic of the crystal phases were detected through quadrupole interactions with Cd daughter nuclei using the method of perturbed angular correlation of gamma rays. Analysis of site fractions of the probes led to two alternative interpretations that could not be distinguished on the basis of the present measurements: 1.) ln probes in the two-phase domain between Ni Al and NiAl segregate to Ni Al with a segregation energy of about 0.10 eV; or 2.) ln probes nucleate embryonic crystals of Ni Al , with the driving force for nucleation being an attraction between In atoms and Ni vacancies that is known to be strong in NiAl