The problem of the reduced dielectric response in thin films of high-permittivity materials is analyzed by studying the soft-mode response in several SrTiO 3 thin films by means of Fourier transform far infrared, monochromatic submillimeter, and micro-Raman spectroscopies. A 300-nm-thick metalorganic chemical vapor deposition film, quasiepitaxially grown on a ͑0001͒ sapphire substrate with a perfect ͗111͘ orientation, displays a ferroelectric transition near 125 K induced by a tensile residual stress, appearing apparently simultaneously with the antiferrodistortive transition. On the other hand, polycrystalline chemical solution deposition films grown on ͑0001͒ sapphire, and also tensile stressed, show a harder soft mode response without the appearance of macroscopic ferroelectricity. This effect, which increases with the film thickness, is explained by a strong depolarizing field induced by the percolated porosity and cracks ͑in the 10-nm scale͒ along the boundaries of columnar grains ͑normal to the probe field direction͒. Brick-wall model calculations showed that 0.2 vol. % of such a porosity type reduces the permittivity from 30000 to less than 1000. The activation of the forbidden IR modes in the Raman spectra in the whole 80-300-K temperature range studied is explained by the effect of polar grain boundaries, in analogy with the bulk ceramics.

Infrared reflectivity spectra of lead zirconate ceramics were measured in the frequency region of 20-3000 cm-1 and temperature range from 10 to 900 K. The data, extended to lower frequencies by coherent source submillimetre, time-domain terahertz transmission and microwave dielectric spectroscopies, were fitted with the factorized model of the dielectric function. Besides the polar optical phonons, one of which slightly softens in the vicinity of the antiferroelectric phase transition, a central-mode type dispersion was revealed in the 5-25 cm-1 (1011-1012 Hz) range. This central mode is found to be responsible for the strong dielectric anomaly in the paraelectric phase, whereas the lattice phonon contribution does not exceed ε'~300 in the vicinity of Tc. We attribute the origin of the central mode to the lattice disorder caused by strongly anharmonic hopping of Pb ions.

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