Pinning effects in incommensurately modulated structures

Abstract

The incommensurate phases of K₂SeO₄-type crystals show thermal hysteresis of dielectric and structural parameters, which has been ascribed to pinning of the modulation. From the temperature dependence of the permittivity, discontinuous processes of the moving lattice of discommensurations are derived and explained by the creation of annihilation of single discommensurations. An analysis of the dielectric measurements demonstrates that the modulation lattice is clearly pinned within the range of the thermal hysteresis and that the coherence length of the modulation parallel to its wavevector must be very small compared with its value in directions perpendicular to it. Furthermore the patterns of discommensurations are stabilised by a defect-induced after-effect and also nominally pure crystals of Rb₂ZnCl₄ show memory effects in the incommensurate state.