Abstract
The instability of thin ferroelectric films is discussed based on the close similarity of dielectric properties between bulk Bi-layered perovskites and thin BaTiO3 films. The dielectric properties of pseudo-two-dimensional layered perovskites suggest that the bulk layered ferroelectric is a good model of ultra-thin ferroelectric film with a few perovskite units, free from any misfit lattice strain. It seems plausible that the ferroelectric interaction is still prominent but shows a crossover from ferroelectric to antiferroelectric along the unique c-axis (perpendicular to the film plane); with decreasing thickness, the ferroelectricity appears within the plane, which results in so-called “canted ferroelectricity”. An extra relaxation mode induced by surface effect of thin films correlates with soft mode, which results in a new intermediate phase between the paraelectric and ferroelectric phases. These evidences may indicate no critical thickness even for ferroelectric ultra- thin films.
Highlights
Ferroelectric compounds exhibit many attractive physical properties such as piezoelectricity, pyroelectricity, high-dielectric constant, non-linear optical effects and bistable nature
The dielectric and structural analogy between Bi-layered perovskites and ferroelectric thin films suggests that the bulk Bi-layered ferroelectrics are a good model of ferroelectric ultra-thin films with a few layers of perovskite units, free from any misfit lattice strain with substrate and surface charges at the interface with electrodes
The soft mode may exist even in ultra-thin films but changes to be highly overdamped near the ferroelectric phase transition temperature Tc
Summary
Ferroelectric compounds exhibit many attractive physical properties such as piezoelectricity, pyroelectricity, high-dielectric constant, non-linear optical effects and bistable nature. Various experiments of ferroelectric thin films have not been enough investigated because of its difficulty to grow a good quality of ultra-thin films and to estimate the effects of misfit strain due to their substrate. It is of fundamental interest in twodimensional structure of ferroelectric thin films experimentally and theoretically since early times [6]. The similarity of dielectric and structural features has been observed between BaTiO3 thin films and Bilayered perovskite compounds This means that both materials have a common mechanism for ferroelectric activity. We will discuss the ferroelectric stability in ultra-thin films in detail from the viewpoint of the nature of Bi-layered perovskites
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