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 $c_{11}$ and $c_{33}$ in the ferroelectric phase shows that these quantities can be approximated by the expression $c_{ii}\sim(T-T_0)^{0.5}$ in the temperature range 0.001K$<T_c-T<$0.2K.