Grain-size effect in BaTiO3 ceramics: study by far infrared spectroscopy

The article offers comparative study of two nanocrystalline (50 and 100nm averaged grain size) and two coarse-grain (1.2 and 10 µm averaged grain size) ceramics by means of Fourier transform infrared spectroscopy in a broad temperature range (10-570 K). Far infrared reflectivity spectra were fitted with the factorized model of the dielectric function and the evaluated dielectric function was compared with the results of low-frequency dielectric measurements. It appears that the stiffening and weakening of the overdamped soft mode is the only reason for the reduced dielectric permittivity of nanocrystalline ceramics in the paraelectric phase, but a strong grain-size dependence of the dielectric constant in the ferroelectric phase is not connected with the changes in lattice dynamics. All single-crystal symmetry changes were detected in all of the samples, but no phase-transition discontinuities was seen in nanocrystalline ceramics. A coexistence of more than one phase is suggested. Strong dependence of the Curie-Weiss temperature on the grain size, earlier revealed by others, is confirmed.