Direct evidence of an order-disorder nature of ferroelectric phase transitions in K_2MgWO_2(PO_4)_2

摘要

K_2MgWO_2(PO_4)_2 single crystal, related to famous optical nonlinear material KTiOPO_4, was investigated by micro-Brillouin scattering with a focus on the central components of the relaxation modes. A critical slowing down has been clearly observed in the vicinity of T_1=436 K while suppressed by the coupling with the order parameter. The temperature dependence of the relaxation time indicates that the former transition is the second order, while the latter is strongly first order. The obtained results show that disorder processes in the sublattice of potassium ions play a major role in the mechanism of these phase transitions.