Triglycine sulfate, TGS

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

Triglycine sulfate, TGS, is ferroelectric below

T_c=322\,\mathrm{K}

. ((a)) A

P

E

hysteresis loop measured on a TGS single crystal at

296\,\mathrm{K}

has saturation branches that extrapolate to

P=2.8\,\mathrm{\mu C\,cm^{-2}}

on the polarisation axis; the loop crosses the polarisation axis (at

E=0

) at

2.5\,\mathrm{\mu C\,cm^{-2}}

, and crosses the field axis (at

P=0

) at

E=1.3\,\mathrm{kV\,cm^{-1}}

. Give the spontaneous polarisation

P_s

, the remanent polarisation

P_r

and the coercive field

E_c

in SI units. ((b)) Above

T_c

, Raman and neutron scattering on TGS find a strong peak centred at zero frequency (a central peak) whose width narrows as

T\to T_c

, with no propagating optic mode ever observed to fall towards zero frequency. Say whether TGS is displacive or order-disorder, and state briefly what is disordering. ((c)) At a temperature above

T_c

the dielectric loss of TGS shows a single relaxational peak with its maximum at

f=15\,\mathrm{GHz}

. Find the associated relaxation time, and say which of the three polarisation mechanisms this relaxation belongs to.

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