ナノ粒子チタン酸バリウムの研究 <学位論文要旨>

Particle barium titanates with various sizes from 10 to 1000 nm in diameter were investigated in the temperature range from around 600 °C to -150 °C using micro Raman spectroscopy. Based on the spectral change, the phase diagram with the particle size versus temperature was determined and the particles can be classified into three regions by their sizes. The first region is the bulk-like region with particle size larger than 90 nm, where the spectra were similar to those of the bulk crystal. The second is the transient region with the size of 60 nm to 90 nm, where new peak was exclusively observed. The last is the highly-distorted region with the size below 60 nm, where no remarkable spectra change was found. The energies of the new peaks in the particles of 70 nm and 85 nm in the transient region have around 145 cm-1, which correspond to the energy at X-point Brillouin zone boundary observed by the inelastic neutron scattering. Thus, it was suggested that the break of the translational symmetry was occurred and the energy at X-point was folded to Γ point. This means the softening of the structure, thus the enhancement of the dielectric constant, so called the size effect, is expected in this region. Moreover, the ferroelectricity is suggested to be retained above around 100 nm particles.