New evidence of the displacive feature of the ferroelectric transition in KDP-type crystals

Employing ab initio calculations we have determined the structural parameters of the paraelectric and ferroelectric phases of KH₂PO₄, which are in good agreement with experiment. The calculations reveal that the O-O bond length and the coordinated motion of the P and K atoms have a large effect on the double-well potential for H, controlling the distance, δ, between the two equilibrium positions of the H along the O-O bond in the paraelectric phase. The calculations provide new evidence that the ferroelectric phase transition in H-bonded crystals has also a displacive feature.