Direct Observation of Tunnelling in KDP Using Neutron Compton Scattering

TL;DR

This study uses neutron Compton scattering to observe proton tunneling in KDP, challenging previous models and providing detailed insights into hydrogen behavior across the ferroelectric transition.

Contribution

First direct measurement of hydrogen momentum distribution in KDP revealing proton coherence and invalidating the order-disorder transition model.

Findings

Proton is coherent over both sites in high-temperature phase.

The Born-Oppenheimer potential aligns with neutron diffraction data.

Vibrational spectrum matches infrared absorption measurements.

Abstract

Neutron Compton Scattering measurements presented here of the momentum distribution of hydrogen in $KH_2PO_4$ (KDP) just above and well below the ferroelectric transition temperature show clearly that the proton is coherent over both sites in the in the high temperature phase, a result that invalidates the commonly accepted order-disorder picture of the transition. The Born-Oppenheimer potential for the hydrogen, extracted directly from data for the first time, is consistent with neutron diffraction data, and the vibrational spectrum is in substantial agreement with infrared absorption measurements. The measurements are sensitive enough to detect the effect of surrounding ligands on the hydrogen bond, and can be used to study the systematic effect of the variation of these ligands in other hydrogen bonded systems.