Second-order terms of the phonon-phason dynamic matrix of an icosahedral quasicrystal: Diffuse intensity and the profile shape around the Bragg peaks - art. no. 144204

摘要

Second-order terms of phonon-phason dynamical matrix of an icosahedral quasicrystal are expressed as polynomial functions of fourth degree with respect to the wave vector components. In terms of this degree there are three independent elastic constants in the phonon block of the dynamical matrix, which is one more than for the case of an isotropic medium. Corresponding peculiarities of acoustic phonons are discussed. Three independent second order elastic constants belong to the phason block and three correspond to the phonon-phason coupling term. Revised theory of the diffuse scattering in the vicinity of Bragg peaks taking into account diffusive nature of phason modes is proposed. Both cases of thermal and quenched phason fluctuations are considered. With taking into account of the second order terms the intensity of the diffuse scattering in the vicinity of Bragg peaks decays likely alpha/(q(2) + betaq(4)), where q is the distance from peaks and coefficients alpha and beta depend on the direction of the q vector. [References: 23]