Communication: Ab initio simulations of hydrogen-bonded ferroelectrics: Collective tunneling and the origin of geometrical isotope effects

Abstract

Ab initio simulations that account for nuclear quantum effects have been used to examine the order-disorder transition in squaric acid, a prototypical H-bonded antiferroelectric crystal. Our simulations reproduce the >100 K difference in transition temperature observed upon deuteration as well as the strong geometrical isotope effect observed on intermolecular separations within the crystal. We find that collective transfer of protons along the H-bonding chains -facilitated by quantum mechanical tunneling -is critical to the order-disorder transition and the geometrical isotope effect. This sheds light on the origin of isotope effects and the importance of tunneling in squaric acid which likely extends to other H-bonded ferroelectrics.Ferroelectric materials have been extensively examined because of their many diverse applications in, e.g., electro-optic, piezoelectric and random access memory devices [1]. Recently interest has intensified in hydrogen (H-) bonded ferroelectrics because of the discovery of above room-temperature ferroelectricity in an organic crystal and the realization that they could potentially be used as cheaper and more environmentally friendly organic electronics [2][3][4]. H-bonded ferroelectrics are also a valuable class of materials through which we can gain deeper understanding of the fundamental nature of H-bonding. Primarily this is because they are wellcharacterized crystalline materials with a range of Hbonding configurations and in most cases have been synthesized in both their standard and deuterated forms.Many H-bonded ferroelectrics exhibit phase transitions to paraelectric phases that lack long-range ordering of protons in the H-bonds. The Curie temperature (T c ) of these transitions can dramatically increase by ∼100 K upon deuteration but the physical origin of this effect is still not fully understood. An early model [5,6] explained this giant isotope effect on the basis of tunneling of protons in double well potentials. However, this model fails to account for experimentally observed geometrical isotope effects in H-bonding geometry between the protonated and deuterated crystals, where H-bonds have been observed to elongate upon deuteration [7,8] -a so called Ubbelohde effect [9,10]. Models involving a coupling between lattice modes and proton dynamics were therefore suggested [11,12]. It was argued on these grounds that tunneling is unnecessary to explain the large increase of T c upon deuteration [7]. On the other hand, neutron Compton scattering experiments on the commonly studied KH 2 PO 4 (KDP) system [13] showed that protons occupy both sites along the H-bonds on a short time-scale * wikfeldt@hi.is above its phase transition at 124 K, indicating coherent quantum tunneling, while no such coherence was found in the deuterated crystal [14]. Theoretical work [15,16] attempted to reconcile these differing interpretations by suggesting that a mechanism behind the Ubbelohde effect may itself be collective tunneling in clusters of atoms in the crystal. However, direct ab init...

Simulation and understanding of atomic and molecular quantum crystals

“…Recently, Wikfeldt and Michaelides (2014) have employed ab initio PIMD simulations based on DFT to investigate the importance of QNE on the atomic ordering and structure of H2SQ (i. e., C 4 H 2 O 4 ). We note that in this case the authors have explicitly considered longrange van der Waals interactions by using a dispersioncorrected DFT functional (see Sec.…”

Inverse Temperature Dependence of Nuclear Quantum Effects in DNA Base Pairs

“…ref. 17,18,[29][30][31]33,[40][41][42][43][44][45][46]. Furthermore by combining PIMD with thermodynamic integration we can explicitly calculate the binding free energy change upon transforming the system from one containing classical nuclei to one containing quantum nuclei.…”

Microscopic Difference of Hydrogen Double-minimum Potential Well Detected by Hydroxyl Group in Hydrogen-bonded System

“…Tunneling in the antiferroelectric-paraelectric transition with regard to the isotope effect in squaric acid was suggested on the basis of path integral molecular dynamics (PIMD) simulations 36 . The ferroelectric and www.nature.com/scientificreports www.nature.com/scientificreports/ paraelectric states of KDP were studied to elucidate the momentum distribution functions (MDF) of correlated proton motion by using PIMD simulations rather than experiments 37 .…”