A two step model for the transformation of decagonal quasicrystals (DQC) to their crystalline approximants is presented.First,a partially disordered approximant domain structure results rapidly from small atomic displacements.subsequently,atomic diffusion leads very slowly to an ordered approximant crystal (AC) structure.The model is discussed on atomic scale.Its basic idea ia to describe the structure of a quasicrystal (QC) as incommensurately modulated structure (IMS).In that description,the average structure (AS) of a QC is at the same time the AS of all its irrational and rational approximants.In this approach, the positional quasicrystal-to-approximant-crystal (QC-AC) transformation can be performed by atomic displacements smaller than any interatomic distance.This mechanism,however,leads inherently to to a certain amount of chemical disorder.In most cases also some positional disorder will result.The tools devleoped for the description of phase transitions of IMS can analogously be used for QC.Examples for one- and two-dimensional quasiperiodic structures are discussed.
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