High temperature phase transitions and metastability in KDP type crystals

Measurements of dielectric constants and infrared reflection were carried out on KH 2 PO 4 , KD 2 PO 4 , and RbH 2 PO 4 single crystals in the temperature range 300 to 500 °K. Both types of measurements show the existence of two distinct phase transitions T p and T 1 in the above range. The T p transition is associated with a change in the hydrogen bond. At this temperature the crystal structure changes from tetragonal to monoclinic. The crystal remains in the new structure even after cooling back past the transition to room temperature. Below T p this phase is metastable and mechanical vibration will release the crystal back to the stable state in room temperature. The T 1 transition seems to be dependent on the PO 1 and the new phase is also methastable below T 1 .