Correlation between hydrogen bond geometry and phase transition temperature in KDP-type ferroelectrics

Molecular dynamics simulation of the modified strong dipole-proton coupling model is used to show that ferroelectric phase transition temperature, Tc, in KH2PO4 (KDP) and KD2PO4 (DKDP) linearly depends on hydrogen bond length, RO−O, and separation between peaks in proton (deuteron) thermal distribution in the paraelectric phase, δ. Separation between lines for KDP and DKDP is larger in the Tc-RO−O dependence than in the Tc-δ dependence indicating equal effect of different RO−O and different proton and deuteron masses on the whole change of Tc between KDP and DKDP and establishing δ as a more important determinant of Tc in these crystals.