Structural Phase Transition Mechanism of Ferroelectrics Thin Films and Nanocrystals

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Thin films of the perovskite ferroelectric PbTiO_3, on silicon single crystal substcrates were prepared by sol-gel processing. Single-layr sol-gel thin films can be deposited with thickness up to 200 nm. The thicker films were produced by multiple deposition. The optimum conditions for obtaining homogeneous and crack free thin films were determined. Far infrared absorption spectra were measured by FTIR spectrometer in the frequency region of 4 - 1000 cm^4. Absorption profiles of spectra of films thicker than 1000 nm are quite similar to those observed in bulk crystals, whereas in thinner films those of absorption lines are rather different. Moreover, the lowest-frequency mode at 80 cm^4, which corresponds to the ferroelectric soft mode in the tetragonal phase, decreases its frequency and intensity as the thickness decreases. The possibility of the occurrescence of stratrural changes around thickness of 1000 nm and 500 nm has been Pointed out.