  This study employs resonant ultrasound spectroscopy (RUS) to investigate the
elastic properties of single crystalline KH$_{2}$PO$_{4}$ (KDP) through the
paraelectric to ferroelectric phase transition. Noteworthy anomalies are
observed in selected resonance modes and their corresponding mechanical quality
factors ($Q$) around the transition temperature. The thermal evolution of
elastic constants ($C_{ij}$) across the phase transition reveals a significant
softening of $C_{11}$, $C_{12}$, and $C_{13}$, accompanied by a stiffening in
$C_{66}$. Additionally, both $C_{33}$ and $C_{44}$ exhibit a minimum value at
the transition. This anomalous behavior of all elastic constants ($C_{ij}$)
across the phase transition suggests the involvement of higher-order coupling
between the lattice and polarization in the KDP crystal. Furthermore, the bulk
modulus ($B$) undergoes a sudden softening precisely at the transition, while
the shear modulus ($G$) initially softens and subsequently stiffens across the
transition.
