Thermal and Nuclear Quantum Effects at the Antiferroelectric to Paraelectric Phase Transition in KOH and KOD Crystals.

Erika Fallacara,Michele Ceotto,Fabio Finocchi,Philippe Depondt,Simon Huppert

doi:10.1021/acs.jpcc.1c06953

Abstract

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low-T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

Highlights

Since its discovery in 1920 by Valasek,[1] ferroelectricity has been a focus of intense research addressed by both academia and industries
In the low-T FE phase, the protons are off-center ordered in the hydrogen bonds (HBs), which are nearly perpendicular to the FE c-axis.[4]
The ≃100 K increase of the Curie temperature upon deuteration has been attributed to the depletion of hydrogen tunneling between the two off-center positions and the weakening of the HBs associated with the FE distortion.[7]

Summary

■ INTRODUCTION

Since its discovery in 1920 by Valasek,[1] ferroelectricity has been a focus of intense research addressed by both academia and industries. As a first step into the crystal structure determination of monoclinic KOH and the description of the HBs, we performed classical geometry optimization at T = 0 K with initial constraints on the positions of the H atoms and identified the following reference static configurations: FE, AFE, and PE. C sin β is tightly correlated with the interlayer O−O distance and to the length of the HBs. The introduction of thermal and quantum effects provides for the IVa phase a better agreement with the experimental data (see Table 1, c sin β at 77 K). All the previous findings are in agreement with experimental results that show an AFE order at low temperatures.[17,18] At 215 K, the free energy barrier for the dipole switching is decreased, allowing the formation of both AFE and FE local arrangements, and a small number of flips occur within the time scale of our simulation. >3.4 Å, a value for which the HBs are almost broken

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES