Phonon-phason coupling in decagonal quasicrystals

Phonon-phason coupling in decagonal quasicrystals has been discussed in terms of phonon strains induced in crystal approximants. Elastically, decagonal quasicrystals can be classified into two groups having different symmetries in the coupling constant tensor; one has only one independent coupling constant, K-4, and the other has two such constants, K-4 and K-5. It has been shown that the orthorhombic and monoclinic approximants formed in various alloy systems can be interpreted as resulting from the two different types of decagonal quasicrystals and that a slight distortion in the unit cell of the monoclinic phases is attributable to the effect of one of the coupling constants, K-5. Phonon strains induced in monoclinic approximants of the Al-Co system have been calculated from their lattice constants and the coupling constants K-4 and K-5 have been evaluated.