Improper ferroelectrics

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Abstract
Ferroelectrics in which the phase transition parameter is not the polarization are discussed from a phenomenological point of view. The concept of improper, pseudo proper and triggered ferroelectric phase transitions is introduced and compared with Aizu's concept of full, partial and faint ferroelectrics. Dielectric, elastic, and electrooptical properties as well as the spontaneous polarization and deformations of improper ferroelectrics are derived from a simplified thermodynamic potential. New examples of improper ferroelectrics are suggested.