Pressure Dependence of Electron Density Distribution of Ferroielectric KNbO3 Polymorphs by Maximum Entropy Method (MEM)
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0987-PP05-03
Pressure Dependence of Electron Density Distribution of Ferroielectric KNbO3 Polymorphs by Maximum Entropy Method (MEM) Using Single Crystal Diffraction Study Takamitsu Yamanaka1, Taku Okada2, Yuki Nakamoto3, and Kenji Ohi4 1
Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonala, 560-0043, Japan
2
Earth and Space Science, Osaka University, 1-1 Machikaneyam, Toyonaka, 560-0043, Japan
3
Kyokugen Extreme Research Center, Osaka University, 1-3 Machikaneyama, Toyonaka, 560-0043, Japan
4
Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka, 560-0043, Japan
ABSTRACT Single-crystal structure analysis of KNbO3 has been executed under high pressure through diamond anvil cell installed in the four-circle diffractometer using synchrotron radiation at Photon Factory, KEK in order to clarify the dielectric property. KNbO3 has three structural transitions with increasing pressure at ambient temperature: from orthorhombic structure with the space group Cm2m (Amm2) to tetragonal structure (P4mm) at about 7.0 GPa, to cubic structure (Pm3m) at about 10.0 GPa. The highest-pressure cubic phase is paraelectric, and the other two phases are ferroelectric. The dielectric changes in KNbO3 are clarified by the successive pressure-change of the electron density distribution observed by maximum entropy method (MEM) using high-pressure diffraction data. The MEM electron density maps suggest that the tetragonal phase designates the largest polarization among three polymorphs. The maps also indicate that the localization of the valence electron around the cation position is more enhanced under higher pressure. INTRODUCTION A large attention has been given to the structure transformations of perovskites from the industrial use of ferroelectrics, ferromagnetics and solid-ionics. The structure changes are often induced from the phonon-electron interaction under compression. Miniature diamond-anvil high-pressure cell (DAC) and multi-anvil high-pressure apparatus have been developed or improved for last two decades. Crystal structure investigations under nonambient conditions are significant subjects for understanding the elastic properties of ferroelectric materials. Raman and infrared spectroscopy also have been executed for these materials. These studies have been performed using single crystal or polycrystalline samples under compression at high temperatures generated by electric resistance heater or laser and at low temperatures using cryostat. Synchrotron radiation (SR) facilities have accelerated high-pressure crystallography, because of their great advantages for diffraction studies at nonambient conditions. Potassium niobate KNbO3 of ferroelectrics has the perovskite-type structures and exhibits the following three structural transitions with decreasing temperature at ambient pressure: from cubic structure (space group Pm3m) to tetragonal structure (P4mm) at 691 K, then to orthorhombic lattice (Cm2m) at 498 K, and finally to rhombohedral (R3m) at 263 K [1]. These transitions are induced
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