Potassium iodide crystallises in the rock-salt structure

PHYSICS · Subjective · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Lattice dynamics and thermal properties of solids · Lattice dynamics and thermal properties of solids

Condensed Matter Physics, Ch Two atoms in the cell: acoustic and optical branches

Problem

Potassium iodide crystallises in the rock-salt structure, with

M_{\rm K}=39.10\ \mathrm{u}

and

M_{\rm I}=126.90\ \mathrm{u}

. Modelling its chain with a force constant

K=12.0\ \mathrm{N\,m^{-1}}

, find the acoustic and optical frequencies at the zone boundary, and the width of the frequency gap between them, in THz.

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