Abstract
The ferroelectric transition in the monoclinic K 2 ZnBr 4 was studied by the molecular dynamics (MD) simulation. Wave-vector dependence of the mode-coupling constant along the c * axis is determined in the paraelectric phase for the transverse optic (TO) and librational (TL) modes which have the A u - and B g -symmetries at the zone center, respectively. The coupling constant takes a maximum at the zone center, showing that there exists strong correlation between the A u and B g modes. Dispersion relations of the TO and TL modes are determined along the c * axis in the paraelectric phase. The frequency of the TO mode decreases toward the zone center from the zone boundary. The TL mode shows also a similar behavior but the dispersion is very small compared to the TO mode. The B g mode keeps its frequency almost invariant over the both paraelectric and ferroelectric phases. In contrast, the frequency of the A u mode decreases fairly as temperature approaches the transition point.
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