Size effect and evidence of a size-driven phase transition in Bi4Ti3O12 nanocrystals

Bi4Ti3O12 nanocrystals (nc-BTO) were prepared by chemical coprecipitation technique. The crystal structure of the nanocrystals covering mean grain sizes from 16 to 48 nm was determined by X-ray diffraction and refined by the Rietveld method. Differential thermal analysis was employed to investigate the temperature dependent phase transition behavior in nc-BTO. With decreasing grain size the tetragonal–orthorhombic phase transition temperature decreases and ferroelectricity weakens. A size-driven phase transition was found at a critical size dc=44 nm for nc-BTO. The high-temperature tetragonal phase stabilizes at room temperature when d<dc. The origin of the structural anomaly is attributed to higher strain energy induced by lattice strain and to higher inter-phase surface energy difference in nc-BTO system.