Path integral Monte Carlo simulations of the geometrical effects in KDP crystals

TL;DR

This paper uses path integral Monte Carlo simulations with simple models to explore how isotope substitution affects the geometry of hydrogen bonds in ferroelectric crystals, revealing quantum tunneling effects that influence structural changes.

Contribution

It introduces a simple, ab initio fitted model to simulate isotope effects and demonstrates how proton tunneling impacts H-bond geometry in KDP crystals.

Findings

Proton tunneling is more efficient than deuteron tunneling in KDP.

Tunneling causes a strong attraction that pulls oxygens together.

The model's results agree with experimental observations of geometrical effects.

Abstract

Path integral Monte Carlo (PIMC) simulations with very simple models were used in order to unveil the physics behind the isotope effects in H-bonded ferroelectrics. First, we studied geometrical effects in the H-bonds caused by deuteration with a general three-site model based on a back-to-back double Morse potential plus a Morse potential between oxygens, fitted to explain different general features for a wide set of H-bonded compounds. Our model results show the Ubbelohde or geometrical effect (GE), i.e., the expansion of the H-bond with deuteration, in agreement to what is observed in H-bonded ferroelectrics with short H-bonds. Moreover, adjusting the potential parameters to ab initio results, we have developed a 1D model which considers the bilinear proton-proton interaction in mean-field to study nuclear quantum effects that give rise to the GE in KDP crystals. PIMC simulations reveal that protons tunnel more efficiently than deuterons along the 1D chain, giving rise to a strong attraction center that pulls the oxygens together. This mechanism, which is based on the correlation between tunneling and geometrial modifications of the H-bonds, leads to a strong GE in the ordered phase of the chain at low temperature which is in good agreement with the experimental data.