Question: Soft modes and lattice transformations sketch a monatomic linear lattice of lattice constant a. (a) Add to each of six atoms a vector to indicate

Soft modes and lattice transformations sketch a monatomic linear lattice of lattice constant a. 

(a) Add to each of six atoms a vector to indicate the direction of the displacement at a given time caused by a longitudinal phonon with wave vector at the zone boundary. 

(b) Sketch the crystals structure that results if this zone boundary phonon becomes unstable (w → 0) as the crystal is cooled through Tc.

(c) Sketch on one graph the essential aspects of the longitudinal phonon dispersion relation for the monatomic lattice at T well above Tc, and at T = Tc. Add to the graph the same information for phonons in the new structure at T well below Tc.

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