Lithium chloride crystallises in the rock-salt structure

PHYSICS · Multi-part · 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

Lithium chloride crystallises in the rock-salt structure, with

M_{\rm Li}=6.94\ \mathrm{u}

and

M_{\rm Cl}=35.45\ \mathrm{u}

. Modelling its chain with a force constant

K=16.0\ \mathrm{N\,m^{-1}}

, find ((a)) the frequency of its zone-centre optical mode, and ((b)) the ratio of the lithium and chlorine amplitudes there, and say which ion moves further.

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