KDP crystal, well known for its low temperature ferroelectric phase transition at -150℃ and also as hydrogen-bonded ferroelectrics, has another phase transition phenomena at around 180℃, which remains puzzling for its origin ever since the first observation of the transition in 1967.
This high temperature phase transition phenomena was experimentally studied through the measurements of dielectric constant; conductivity and the dependence of the dielectric constant on the measuring frequency.
The results show that the high temperature phase transition is mainly due to hydrogen bond weakening at the transition temperature and suggests the possibility of contribution from the hindered rotations of $PO_4$ tetrahedral units in KDP.
