APPLICATION OF THE CLUSTER VARIATION METHOD TO WAVE-NUMBER DEPENDENT SUSCEPTIBILITY FOR KDP-TYPE FERROELECTRICS

The cluster variation method (CVM) in the cactus approximation is appl ied to a pseudo-spin Ising Hamiltonian of the Slater-Takagi model for KDP-type hydrogen-bonded ferroelectrics to calculate the wave-number d ependent susceptibility. After a microscopic Landau-Ginzburg variation al free energy is constructed, the wave-number dependent susceptibilit y is derived from an equation of state obtained as the thermodynamic e quilibrium condition under an inhomogeneous electric field. A strong s uppression of fluctuations along the easy (z) axis is shown to appear as the ice-rule limit is approached. Though the present method is diff erent from that of Havlin ct al., the remarkable resemblance can be se en in both expressions of susceptibility.