Potassium dihydrogen phosphate (KDP) undergoes a paraelectric (PE) to ferroelectric (FE) phase transition at 123 K. The primitive cell containing two formula units changes from the tetragonal symmetry (I42d) in the PE phase, to the orthorhombic symmetry (Fdd2) in the FE phase. The optical fourfold c axis of the PE phase, becomes the ferroelectric axis below 123 K.
B. Wyncke, B. Wyncke,
"Phonon modes in KH2PO4 - type crystals", Proc. SPIE 1576, 16th International Conference on Infrared and Millimeter Waves, 15767F (4 December 2017); doi: 10.1117/12.2297980; https://doi.org/10.1117/12.2297980