ペロブスカイト型結晶における遂次相転移

概要

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Successive phase transitions in perovskite crystals ABX3 due to condensations of Γ15 and R25-M3 modes at the center and R-M corners of Brillouine zone are described and the following three characteristics are discussed. 1) An alienating property between the condensation of Γ15 mode and those of the zone boundary R25-M3 modes is noticed to exist except SrTiO3. Such property is interpreted as due to coupling constant between the B-ion and X6-octahedron in Γ15 soft mode. 2) The condition for successive and nonsuccessive phase transition observed in these crystals is characterized by a ratio √‹u2›/ΔZ2, where ‹u2› is the mean square ionic displacement due to thermal vibration and ΔZ is the ionic shift in condensed phase. 3) The extremly anisotropic diffuse scatterings found in X-ray and electron diffraction photgraphs can be regarded as a premonitory property indicating the direction of ionic shifts which appear immediatly after the soft mode is condensed.
日本結晶学会の論文