Structural change driven by hidden modes - Ferrielectricity and ferrielasticity in AA'BX4 type crystals

Some AA'BX4 type crystals exhibit ferrielectric and/or ferrielastic propert ies. For the mechanism, two cases should be separately considered; (a) the polarization or strain is modulated in the superlattice, or (b) the crystal consists of sublattices, which are competing with each other. As a typical example, ferroelectric phase IV and ferroelastic phase V of LiRbSO4 are di scussed in detail. In the former phase, normal modes with k(z)=2/5 and 1/5 should freeze simultaneously to produce the polarization. A strong field in duces another commensurate phase with k(z)=1/3, where a larger polarization is realized; thereby a triple hysteresis loop is observable. In the latter phase, a new mechanism of ferrielasticity is proposed; a hidden A(g) mode makes the monoclinic angle passing through 90 degrees continuously.