Peculiarities of the phase transitions of the perovskite-type structure thin ferroelectric films

The heteroepitaxial ferroelectric thin films of (Ba0.85Sr0.15)TiO3 were deposited on (001) fractures of MgO single crystals by r.f. sputtering. The films are characterized by the perovskite-type structure with clear tetragonal deformation and complete parallel orientation in relation to the substrate. The parameters of the elementary cell as well as character of the domain structure and dielectric parameters show strong dependence on the thickness of the thin films (thicknesses within the range d(f)=(10-5000) x 10(-9) m). The thin films of a very small thickness (d(f) < 20 x 10(-9) m) have the 90 degrees a-a-domain structure which transits into the c-domain structure when the thickness increases. The permittivity epsilon of the thin film and the remanent polarization P-r decrease when the thickness of the film increases. The temperature T which corresponds with the transition P4mm <----> Pm3m shifts towards higher temperatures, whereas the temperature T which corresponds with the transition Amm2 <----> P4mm reduces when the thickness of the thin film increases. The theoretical model describing anomalous properties of the heteroepitaxial (Ba, Sr)TiO3 thin films has been developed.