Comparative Study of Phase Transitions in Polycrystalline and Epitaxial BaTiO₃Thin Films by Specific Heat Measurements

The experimental data on the thermal properties of the nanostructured perovskite ferroelectrics are presented and analysed. The ability of the modified 3ω method for specific heat measurements is discussed and illustrated by the thermal properties of the thin polycrystalline (BaTiO 3 on fused SiO 2 , 50–1100 nm) and epitaxial (BaTiO 3 on MgO, 50–500 nm) films. The sharp decrease of the transition temperature, surplus entropy and spontaneous polarization were obtained for the polycrystalline films. The critical thickness and crystallite size are estimated for this case from the variation of both crystallite sizes at constant thickness and film thickness at constant crystallite size. The similarity of specific heat, phase transition excess entropy evolution in polycrystalline films and nanograined ceramics is revealed. The data on the specific heat of epitaxial films show the extremely diffused anomaly near 400 K, the phase transition has the weak tendency of shifting to higher temperatures with the decreasing of the film thickness. No anomalies were detected for the thinnest (50 nm) film up to 480 K.