Deuteration removes quantum dipolar defects from KDP crystals

TL;DR

This study uses all-atom path integral molecular dynamics to reveal how deuteration removes quantum dipolar defects in KDP crystals, explaining differences in their ferroelectric properties.

Contribution

It provides a microscopic model showing how proton tunneling creates defects in KDP, which are absent in DKDP, elucidating isotope effects on ferroelectric behavior.

Findings

Proton tunneling generates non-polar phosphate configurations in KDP.

Deuteration suppresses quantum dipolar defects, leading to classical behavior in DKDP.

Quantum fluctuations influence the ferroelectric transition and residual entropy.

Abstract

The structural, dielectric, and thermodynamic properties of the hydrogen-bonded ferroelectric crystal potassium dihydrogen phosphate ($\mathbf{KH_2PO_4}$), KDP for short, differ significantly from those of DKDP ($\mathbf{KD_2PO_4}$). It is well established that deuteration affects the interplay of hydrogen-bond switches and heavy ion displacements that underlie the emergence of macroscopic polarization, but a detailed microscopic model is missing. Here we show that all-atom path integral molecular dynamics simulations can predict the isotope effects, revealing the microscopic mechanism that differentiates KDP and DKDP. Proton tunneling in the hydrogen bonds generates phosphate configurations that do not contribute to the polarization. These dipolar defects are always present in KDP, but disappear at low temperatures in DKDP, which behaves more classically. Quantum disorder confers residual entropy to ferroelectric KDP, explaining its lower spontaneous polarization and transition entropy relative to DKDP. Tunneling should also contribute to the anomalous heat capacity observed in KDP near absolute zero. The prominent role of quantum fluctuations in KDP is related to the unusual strength of the hydrogen bonds in this system and should be equally important in the other crystals of the KDP family, which exhibit similar isotope effects.