An oxide perovskite well above its ferroelectric transition ( ) has and

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

An oxide perovskite well above its ferroelectric transition (

T\gg T_c

) has

\varepsilon_s=310

and

\varepsilon_\infty=5.4

. Infrared reflectivity shows a single sharp resonance (not a relaxational peak) at

\nu_{TO}=2.70\,\mathrm{THz}

(

11.2\,\mathrm{meV}

), and no feature at lower frequency. ((a)) Say which of the three polarisation mechanisms is responsible for the plateau between

\varepsilon_\infty

and the resonance, and which is responsible for

\varepsilon_\infty

itself. ((b)) Use the Lyddane-Sachs-Teller relation to find

\nu_{LO}

. ((c)) State whether the resonant (rather than relaxational) character of the

2.70\,\mathrm{THz}

feature is more consistent with a displacive or an order-disorder route to the transition, and what would confirm this on cooling.

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