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신선협,고재현,원충재,허남정 한국물리학회 2015 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.66 No.6
The elastic properties of BaxSr1−xTiO3 single crystals with x = 0.79 were investigated by usingBrillouin spectroscopy. The temperature dependences of the longitudinal and the transverse acousticphonon modes were obtained and revealed three structural phase transitions at ∼47, −31, and −88◦C. The longitudinal acoustic phonon mode showed softening in the paraelectric phase, whichbecame more substantial at temperatures between 100◦C and the ferroelectric phase transitiontemperature. This was accompanied by a significant increase in the acoustic damping. Theseacoustic anomalies were attributed to the order-disorder character of the phase transition and tothe dynamics of associated precursor polar clusters.
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고재현,신선협,장문규,원충재,허남정 한국물리학회 2018 Current Applied Physics Vol.18 No.1
The acoustic behaviors of oxygen-reduced barium titanate (BaTiO3-d) single crystals with d~0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO3 (BTO) showed two pronounced anomalies at approximately 112 C and 11 C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 C compared to those of the pure BaTiO3 suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO3 were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.
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