APPLICATION OF THE CLUSTER VARIATION METHOD TO WAVE-NUMBER DEPENDENT SUSCEPTIBILITY OF THE KDP-TYPE FERROELECTRICS ABOVE AND BELOW THE TRANSITION-TEMPERATURE

The cluster variation method (CVM) in the cactus approximation is appl ied to the Slater-Takagi model for KDP-type hydrogen-bonded ferroelect rics to calculate the wave-number dependent susceptibility chi(q), mai nly focussing on the ferroelectric phase. The strong suppression of po larization fluctuations along the easy z-axis is shown to appear not o nly in the paraelectric phase but also in the ferroelectric phase when the ice-rule limit is approached. An analytical expression of the spo ntaneous polarization in the present approximation is fully utilized f or calculations on chi(q) below the transition temperature.