Ab Initio Studies of H-bonded Systems: the Cases of Ferroelectric KH2PO4 and Antiferroelectric NH4H2PO4

S. KOVAL; J. LASAVE; R.L. MIGONI; J. KOHANOFF; N. S. DALAL

Ferroelectrics - Characterization and Modeling

Intech

Lugar: Rijeka; Año: 2011; p. 411 - 436

The purpose of the present contribution is to review and discuss the fundamental behavior of the FE and AFE H-bonded materials KDP and ADP, as explained by our recent first principles calculations. The following questions are addressed: (i) What is the microscopic mechanism leading to ferroelectricity in KDP and antiferroelectricity in ADP?, (ii) What is the quantum mechanical explanation of the double-site distribution observed in the PE phases of KDP and ADP?, (iii) How do deuteration produce geometrical effects?, (iv) What is the main cause of the giant isotope effect: tunneling, the geometrical modification of the H-bonds, or both? It is shown that the H off-centering controls the instability proccess in KDP. This ordering leads to an electronic charge redistribution and ionic displacements that originate the spontaneous polarization of the FE phase. On the other hand, the origin of antiferroelectricity in ADP is ascribed to the optimization of the N-H· · ·O bonds. The closeness in energy found between the AFE and the hypothetical FE phases in ADP is in accordance with the experimental observation of coexistence of AFE and FE microdomains near the vicinity of the transition. The dynamics of protons alone in the PE phases of KDP and ADP cannot explain the observed double-peaked proton distribution in the bridges. By contrast, the importance of the correlations between protons and heavier ions displacements within clusters has been demonstrated. Recent evidence of tunneling obtained from Compton scattering measurements support our conclusions regarding the existence of tunneling clusters. We also show that the huge isotope effect observed in KDP cannot be explained by the quantum effects of a mass change obtained in a system at fixed geometry and potential. We found that as a consequence of the modification of the covalency in the bridges, structural changes arise producing a feedback effect on the tunneling that strongly enhances the phenomenon. The resulting amplification in this nonlinear feedback of the geometrical effect is in agreement with experimental data from neutron scattering.