Elastic properties of KH2PO4 at the ferroelectric phase transition

We report results of ultrasonic studies of a single crystal of KDP in the temperature range 2–300 K. The longitudinal and transverse sound velocities along [001] and [100] directions were obtained as functions of temperature. The analysis of the temperature evolution of pseudomoduli c11 and c33 in the ferroelectric phase shows that these quantities can be approximated by the expression c ii ~ ( T − T 0 ) 0.5 in the temperature range 0.001 K < T c − T < 0.2 K .