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Abstract
Temperature dependence of the mean spontaneous polarization and mean relative permittivity were calculated by modified Devonshire theory considering the finite volume of coherently polarising regions. In the regions of less than 15.6 nm3 as a volume (2.5 nm in each length), thermal fluctuation of the spontaneous polarization became significant and the mean spontaneous polarization reduced. Such behaviours resulted in the increase below Curie point and the decrease around Curie point of relative permittivity, respectively.
