抄録
The mode-coupling between the rotational and translational motions in the monoclinic K2ZnBr4 was studied by the molecular dynamics simulation and X-ray structure analysis. In the structure analysis, the Fourier analysis indicates that, in the paraelectric phase, electron densities of the bromines Br1 and Br2 on the mirror plane are fairly elongated in the b direction, while the density of the bromine Br3 at the general position spreads more or less in the a direction. In the ferroelectric phase, the elongation is suppressed in particular for Br1 and Br2, and the densities of Br3 and Br4, which are equivalent each other in the paraelectric phase, are nearly isotropic. In addition, Br1 and Br2 displace in the b direction through the rotation of the ZnBr42− ion about the a axis. In the simulation, the ZnBr42− ions are treated as rigid-bodies. The trajectories of the bromines reproduce satisfactorily the characteristic feature of the Fourier maps. This means that the ZnBr42− ions are approximately regarded as rigid-bodies even in the real K2ZnBr4. The mode-coupling analysis shows that, in the ZnBr42− rigid-bodies, the rotational motion about the a axis and the translational motion in the b direction couple strongly. Moreover, the displacements of rotational and translational motions in the b direction are almost synchronous for Br1 and Br2, and almost asynchronous for Br3 and Br4.
