Ferroelectric and Ferroelastic Phase Transformations in Which the Soft Waves Have a Wavelength Equal to a General Rational Number Times the Lattice Constant

Abstract

Ferroelectric phase transformations and ferroelastic ones in which the soft vibration modes have a wavelength equal to a general rational number λ/μ (λ, μ: integers) times the relevant lattice constant are theoretically investigated. As an example it is assumed that the space group of the prototypic phase is Pnam and that the wave normal of the soft vibration modes is perpendicular to the symmetry planes n. It is found out how λ and μ influence the dielectric and elastic properties and the space group of the phase resulting from condensation of the soft waves.