Phonons and origin of polarization in KDP crystals (poster)

G. COLIZZI; J. LASAVE; R. L. MIGONI; J. KOHANOFF

Xi'an, China

Congreso; 12th International Meeting on Ferroelectricity; 2009

Xi'an Jiaotong University

The lattice dynamics of hydrogen-bonded KH2PO4 (KDP) and its deuterated analog DKDP is studied via first-principles linear response calculations. Two types of instabilities are observed in the prototype paraelectric (PE) phase at the Brillouin-zone center. One is a normal mode polarized along the tetragonal axis of the PE phase, and gives rise to the spontaneous polarization of the low-temperature ferroelectric (FE) phase. The other instability corresponds to a two-fold degenerate mode orthogonal to that axis.  Such instability is related to the formation of metastable Slater defects in the FE phase, as suggested from experiments and verified by recent ab-initio calculations.[1] Other relevant phonon modes are analyzed, especially those that couple to the FE mode when this latter performs large excursions, in particular a mode describing the rotation of the PO4 tetrahedra, and also a temperature-independent Raman mode at low frequencies. For the FE mode we calculate the Born effective charges, and show that the origin of polarization can be traced back to a large off-diagonal component of the Born effective charge tensor of the protons. This supports the view that proton off-centering is at the origin of ferroelectricity,[2] but now it appears to be directly responsible for the polarization, while the P atom displacements give a minor contribution.