KCl crystallises in the rocksalt structure

PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Dielectric and optical properties of solids · Dielectric and optical properties of solids

Solid State Physics, Ch Dielectric response, local fields and ferroelectricity

Problem

KCl crystallises in the rocksalt structure, with a conventional cubic cell of side

a=6.29\,\mathrm{\AA}

containing 4 K

^+

^-

ion pairs. Tabulated electronic polarisability volumes (defined so that the actual polarisability is

\alpha=4\pi\varepsilon_0\alpha_p

) are

\alpha_p(\mathrm{K^+})=1.33\,\mathrm{\AA^3}

and

\alpha_p(\mathrm{Cl^-})=2.97\,\mathrm{\AA^3}

. ((a)) Use the Clausius-Mossotti relation to predict the high-frequency (electronic) dielectric constant

\varepsilon_\infty

and the refractive index

n

of KCl. ((b)) The measured values are

\varepsilon_\infty=2.19

and

n=1.49

. State the percentage by which the prediction differs from the measured

\varepsilon_\infty

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