Phonons, Instabilities and Origin of Polarization in KDP Crystals

Abstract

The Γ-phonons of KH₂PO₄ (KDP) and its deuterated analog DKDP are studied via first-principles linear response calculations. The paraelectric phase shows two instabilities. One for a z-polarized mode, which leads to the spontaneous polarization P_s of the ferroelectric phase. The other corresponds to a two-fold degenerate xy-polarized mode. Other phonons are analyzed, which couple to the ferroelectric one at large amplitudes and are relevant for the ferroelectric transition. We show that P_s is mainly of electronic nature, since it arises mostly from an off-diagonal component of the Born effective charge tensor of H, with minor contribution from P atoms displacements.

Keywords:

• Phonons
• ferroelectricity
• KDP