In physics, a phason is a form of collective excitation found in aperiodic crystal structures.
As a result of this rearrangement, or modulation, the waves that describe the position of atoms in the crystal change phase -- hence the term "phason".
In the language of the superspace picture commonly employed in the description of aperiodic crystals in which the aperiodic function is obtained via projection from a higher dimensional periodic function, the 'phason' displacement can be seen as displacement of the (higher-dimensional) lattice points in the perpendicular space.
In incommensurately-modulated crystals, phasons may be constructed from a coherent superposition of phonons of the unmodulated parent structure, though this is not possible for quasicrystals.
[5] Therefore, metastable quasicrystals grown by rapid quenching from the melt exhibit built-in phason strain[6] associated with shifts and anisotropic broadenings of X-ray and electron diffraction peaks.