The intrinsic ferroelectric phase transitions occurring in perovskite-structure epitaxial films of Pb 0.5 Sr 0.5 TiO 3 are experimentally assessed and studied. The low-frequency dielectric response of thin-film heterostructure capacitors, polycrystalline films, and prototype ceramics is analyzed using phenomenological and equivalent circuit approaches. To explore the paraelectric to ferroelectric phase transitions, the derivative of inverse permittivity is used. In ceramics, the transition is of first order. In the epitaxial films, the order of the intrinsic transitions is difficult to determine in terms of classical phenomenology. Based on analysis of such factors as epitaxial strain, internal electric field, and domain configurations, we suggest that the configuration and dynamics of ferroelectric domains are responsible for this. The origin of apparently relaxor-looking dielectric peaks is discussed.
Despite wide studies of Na0.5Bi0.5TiO3, structure of this material and its connection with the observed physical properties still raise numerous questions due to mutually contradicting results obtained. Here, structure and dielectric properties of poled and unpoled Na0.5Bi0.5TiO3-CaTiO3 solid solutions are studied, projecting the obtained concentration dependence of structure and dielectric properties on pure Na0.5Bi0.5TiO3 as the end member of this material group. X-ray diffraction patterns for Na0.5Bi0.5TiO3-CaTiO3 solid solutions reveal dominating of an orthorhombic Pnma phase, even for the compositions approaching the end composition (Na0.5Bi0.5TiO3), whereas structure of pure Na0.5Bi0.5TiO3 can be considered, assuming coexistence of rhombohedral and orthorhombic phases. This allows one to avoid appearance of a large difference of rhombohedral distortions between the unpoled and poled Na0.5Bi0.5TiO3, if the rhombohedral distortion is calculated as for single R3c phase. Features of dielectric permittivity, corresponding to the observed structural phase transition, are identified. It is discussed that the rhombohedral R3c phase is responsible for appearance of the frequency-dependent shoulder of dielectric permittivity temperature dependence, characteristic for unpoled Na0.5Bi0.5TiO3.
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