Phase transformation of BaTiO3 nanoparticles synthesized by RF-plasma CVD

The present paper reports an investigation on the size-dependent cubic-tetragonal phase transformation of barium titanate (BaTiO3) using BaTiO3 nanoparticles prepared by radio-frequency plasma chemical vapor deposition (RF-plasma CVD) as starting materials. The particle sizes of BaTiO3 were controlled by heat treatment of as-prepared BaTiO3 nanoparticles at various temperatures. The BaTiO3 nanoparticles under 30 nm (=dSEM: D50 of particle size distribution obtained from SEM image) were assigned to a perfect cubic structure using Raman spectroscopy and Rietveld analysis. In the range from 30 to 70 nm (dSEM), the stable phase at room temperature gradually transformed to a tetragonal structure, which suggests that the crystal structure in this region is pseudo-cubic. Considering the reliability factor of the Rietveld analysis, the phase transformation from cubic to tetragonal was confirmed to have occurred above 70 nm (dSEM). The products of 100 nm (dSEM) showed tetragonality (lattice constant ratio of the c-axis to the a-axis in perovskite unit cell) of 1.008 by Rietveld analysis and an endothermic peak in the DSC profile due to the tetragonal-cubic transformation at the Curie temperature. This suggests that a clear tetragonal structure can be observed above 100 nm.