BaTiO$_{3}$ nanocrystals studied by Raman spectroscopy

Variable-temperature Raman spectroscopy study of BaTiO$_{3}$ nanocrystals of varied sizes (from 8 to 20 nm) will be presented. Highly uniform cube-shaped BaTiO$_{3}$ nanocrystals have been prepared by solvothermal synthesis at temperatures below 140\r{ }C and characterized by x-ray diffraction and transmission electron microscopy. Raman spectra (measured with ultraviolet and visible excitation) show that all nanocrystals studied are ferroelectric; nanocrystal size effect on the Curie temperature has been investigated. Temperature evolution of Raman spectra (10--600K) demonstrates that the ferroelectric phase of the nanocrystals is different from the bulk BaTiO$_{3}$. The transitions from tetragonal to orthorhombic and from orthorhombic to rhombohedral phases, which are characteristic for bulk BaTiO$_{3}$, have not been observed in the nanocrystals; the ferroelectric phase in the nanocrystals is the same in the entire temperature range below $T_{c}$, and is different from any of the bulk phases. The observed behavior may be explained by complex polarization patterns theoretically predicted for zero-dimensional ferroelectrics.