Thermal expansion of nano-sized BaTiO3†
BaTiO3 (BTO) nanoparticles (NPs) with various sizes from 6 to 210 nm were synthesized by a one-step hydrothermal method. The powder X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) display the formation of highly crystalline BTO NPs with a narrow size distribution. The size effect of BTO NPs on crystal structures and thermal expansion was investigated according to refined high-temperature XRD data obtained through a full-profile Rietveld method. These results show that unit cell volume (UCV) drastically increases and tetragonality (the c/a ratio) decreases as the size of BTO NPs is reduced. The coefficients of thermal expansion (CTE) of different BTO NPs reveal that the size dependence of positive thermal expansion of BTO mainly results from the ferroelectrostriction of the tetragonal phase. Through further analysis, we find that the mixed phase of tetragonal and cubic BTO may be a core/diffused shell structure which changes with decreasing NP size. Furthermore, it is speculated that the core/shell structure or tetragonality disappears when the size of BTO NPs is reduced to about 12 nm. The value is in accordance with the obtained results through the study of the change of Raman spectra and the ratio of c/a with the size of BTO NPs as well as with the reported reasonable results in the range of 5–16 nm.