CRITICAL-BEHAVIOR OF SPECIFIC-HEAT IN FERROELECTRIC K2ZNCL4 AND (NH4)2BEF4 AT NORMAL-INCOMMENSURATE PHASE-TRANSITION

Measurements of specific heat were carried out on structurally incomme nsurate ferroelectrics K2ZnCl4 and (NH4)2BeF4 over a wide temperature region with an AC calorimetric technique. Critical indices alpha assoc iated with the normal-incommensurate phase transition are obtained as 0.12+/-0.02 for K2ZnCl4 and 0.18+/-0.02 for (NH4)2 BeF4. These values do not agree with that in the 3 d XY-model proposed by Bruce and Cowle y, and are rather comparable with alpha = 0. 125 for the 3d Ising-mode l. A crossover from a logarithmic behaviour to a power-law behaviour w as found at reduced temperatures t(cr) = 1 X 10(-3) for K2ZnCl4 and 5 x 10(-3) for (NH4)2BeF4, respectively.