Finite-size effects of lattice structure and soft mode in bismuth titanate nanocrystals

摘要

Bismuth titanate nanocrystals with grain size from 25 to 83 nm were synthesized by chemical coprecipitation via heat treatment. Lattice structures of the nanocrystals were determined by X-ray diffraction and refined by using the Rietveld method. An empirical relationship a/b similar to 1 - e(-d) is established between the orthorhombic distortion ratio a/b and the grain size d, from which a critical grain size d(c) for vanishing of ferroelectricity of bismuth titanate nanocrystals was found to be 38 nm. Low-frequency Raman spectra were investigated at various grain sizes in the nanocrystals. The results show that the soft mode obviously softens and ferroelectricity weakens with decrease in the grain size. An empirical formula omega(2) = omega(2)(0) (1 - d(0)/d) is found to describe the relationship between the soft-mode frequency omega and the grain size d with two fitting parameters omega(0) = 29 cm(-1) and d(0) = 23 nm. (C) 2007 Elsevier Ltd. All rights reserved.