First-principles Studies of Hydrogen-bonded Ferroelectrics and Antiferroelectrics

S. KOVAL

La Plata

Workshop; 4th Workshop on Novel Methods for Electronic Structure Calculations & First Southamerican Congress on Materials; 2011

Universidad de la Plata, Argentina

p { margin-bottom: 0.21cm; } p { margin-bottom: 0.21cm; } Potasium dihydrogen phosphate (KH2PO4 or KDP) is the prototype member of the family of H-bonded ferroelectrics whose phase transitions are connected with the proton off-centering along their H-bonds. A striking feature observed in these materials is the huge isotope effect on Tc upon deuteration. This effect was first explained by Blinc et al.1 in 1960, based on the idea that the hydrogen tunnels between two equivalent off-centered positions in the O-H···O bridge between PO4 tetrahedra. However, neutron diffraction experiments carried out decades after showed linear correlation between Tc and the geometrical modification of the H-bonds upon deuteration.2 Thus, it seems that proton and host cage are connected in a non-trivial way and are not separable, suggesting a deep revision of the existing theories.3 By means of ab-initio DFT calculations we studied the ferroelectric phase transition and the associated isotope effect in KDP.4,5 We found that tunneling is allowed only for clusters involving correlated protons and heavy ion displacements. The sole effect of mass change upon deuteration is not sufficient to explain the huge isotope effect. Instead, we find that structural modifications deeply connected with the chemistry of the H-bonds produce a feedback effect on tunneling that strongly enhances the phenomenon.4 The resulting influence of the geometric changes on the huge isotope effect agrees with the neutron scattering data. Thus, the aspects of tunneling and geometric effects, which were largely debated and considered in the past as unique origins of the giant isotope effect, appear in our calculations as complementary and deeply connected to each other.5 By analogous first-principles DFT calculations, we have studied the origin of antiferroelectricity in the isomorphous compound ADP, where A (NH4) substitutes K in the structure of KDP.6,7 Our calculations show that the antiferroelectric order is favored over the ferroelectric one by the optimization of the N-H···O bonds and the concomitant ammonium distortions. ADP shows also a huge isotope effect upon deuteration. The feedback effect between tunneling and structural modifications observed in the H-bonded ferroelectrics is a phenomenon of wider implications, particularly important in many biological processes. 1 Blinc R.; J. Chem. Solids 13,204 (1960) 2M.I. McMahon, R.J. Nelmes, W.F. Kuhs, R. Dorwarth, R.O. Piltz, and Z. Tun McMahon et al.; Nature 348, 317 (1990) 3 J.A. Krumhansl, Nature (London) 348, 285 (1990). 4 Koval S., Kohanoff J., Migoni R. and Tosatti E.; Phys. Rev. Lett. 89, 187602 (2002) 5 S. Koval, J. Kohanoff, J. Lasave, G. Colizzi, and R.L. Migoni, Phys. Rev. B 71, 184102 (2005). 6 J. Lasave, S. Koval, N.S. Dalal, and R.L.Migoni, Phys. Rev. Lett. 98, 267601 (2007) 7 J. Lasave, S. Koval, R. L. Migoni, and N.S. Dalal, J. Chem. Phys. 135, 084504 (2011)