Ferroelectric and antiferroelectric dynamics of pseudo‐one‐dimensional CsD2PO4

A simple model is proposed for the pseudo‐one‐dimensional (1d) ferroelectric and antiferroelectric phase transitions in CsD2PO4 which is based on the behavior of spin 1/2 Ising chains in a longitudinal molecular field. The observed phase diagram can be reproduced if one assumes that the coupling constant beween ’’unlike’’ chains changes its sign when the hydrostatic pressure exceeds a critical value. In addition to the usual ’’critical’’ slowing down of the order parameter fluctuations an ’’anisotropy’’ slowing down is predicted which may explain the fact that the soft modes in 1d CsH2PO4 and PbHPO4 are much slower than in 3d KH2PO4 and KD2PO4.