Interplay between proton ordering and ferroelectric polarization in H-bonded KDP-type crystals

The origin of ferroelectricity in KH2PO4 (KDP) is studied by first-principl es electronic structure calculations. In the low-temperature phase, the col lective off-centre ordering of the protons is accompanied by an electronic charge delocalization from the near and localization at the far oxygen with in the O-H...O bonds. Electrostatic forces. then, push the K+ ions towards off-centre positions, and induce a macroscopic polarization. The analysis o f the correlation between different geometrical and electronic quantities, in connection with experimental data. supports the idea that the role of tu nnelling in isotopic effects is irrelevant. Instead, geometrical quantum ef fects appear to play a central role. (C) 2001 Elsevier Science B.V. All rig hts reserved.