A close look at the microscopic mechanisms of H-bonded ferroelectrics

S. KOVAL; J. KOHANOFF; R. L. MIGONI

Trieste

Workshop; Tenth International Workshop on Computational Material Science: Total Energy and Force Methods, ICTP; 2001

ICTP, Trieste, Italia

Abstract:First-principles calculations for KH 2 PO 4 (KDP) show that hydrogen off-center (OC) ordering in the ferroelectric phase is accompanied by electronic charge delocalization from the acceptor and localization at the donor oxygen within the O-H···O bonds. This induces P atoms off-centering in the PO 4 tetrahedrons and pairing with K + ions along the tetragonal axis, thus developing macroscopic polarization. Centered H are collectivelly unstable. However, if the protons are centered, the P and K atoms are stable in their centered positions. In addition, off-centering of a single proton is not energetically favorable. Only correlated proton motions, involving also displacements of heavier atoms, exhibit significant double wells, thus questioning the role of tunneling in isotope effects.