DYNAMICAL BEHAVIOR OF APERIODIC INTERGROWTH CRYSTALS

Abstract

An overview is given of the current knowledge pertaining to the sliding mode in incommensurate intergrowth compounds. It is found that both theoretical understanding and experimental information is wanting. Responsible for the latter is the lack of high-quality, very pure crystals of incommensurate intergrowth compounds, thus destroying the sliding mode by the effect of pinning. A phenomenological description is presented of the sliding mode in the long wavelength limit, using an extension of the theory of elasticity. The six by six elastic tensor is extended to a nine by nine elastic tensor describing the elastic waves in (3+1)-dimensional aperiodic crystals. The equations of motion are solved and symmetry restrictions on the components of this elastic tensor are given for all superspace groups. Experimental information as obtained by Brillouin scattering, inelastic neutron scattering and X-ray diffraction is discussed against the background of the present theoretical knowledge. Particular attention is given to pinning, to damping and the existence of propagating and overdamped regimes, and to the polarization of the sliding mode.