Defect structure in aliovalently-doped and isovalently-substituted PbTiO3 nano-powders

Abstract

The defect structure of Fe3+-, Cu2+-, Mn4+- and Gd3+-doped PbTiO3 nano-powders has been studied by electron paramagnetic resonance (EPR) spectroscopy. Analogous to the situation for 'bulk' ferroelectrics, Fe3+ and Cu2+ act as acceptor-type functional centers that form defect complexes with charge-compensating oxygen vacancies. The corresponding defect dipoles are aligned along the direction of spontaneous polarization, PS, and possess an additional defect polarization, P-D. Upon the transition to the nano-regime, the defect structure is modified such that orientations perpendicular to P-S, (Fe-Ti'-V-O(center dot center dot))(perpendicular to)(center dot) and (Cu-Ti ''-V-O(center dot center dot))(perpendicular to)(x) also become realized. Moreover, the binding energy for the defect complexes is lowered such that instead 'free' Fe-Ti' and V-O(center dot center dot)-centers are formed. As a consequence, the concentration of mobile V-O(center dot center dot) that enhances the ionic conductivity through drift diffusion is increased for the nano-powders. Finally, in the nano-regime the ferroelectric 'hardening' is expected to be considerably decreased as compared to the 'bulk' compounds. In contrast to the acceptor-type dopants, the donor-type Gd3+ dopant is incorporated as an 'isolated' functional center, where charge compensation by means of lead vacancies is performed in distant coordination spheres.