Hydrogen modes in KH$_2$PO$_4$ under pressure from ab initio calculation and inelastic neutron scattering

TL;DR

This study combines ab initio calculations and inelastic neutron scattering to analyze how pressure affects hydrogen modes in KH$_2$PO$_4$, revealing that bending modes dominate the phonon triplet observed.

Contribution

It provides a combined theoretical and experimental analysis clarifying the origin of phonon triplets in hydrogen-bonded materials under pressure.

Findings

OH-bending modes harden with pressure

OH-stretching modes soften with pressure

Phonon triplet mainly due to bending modes

Abstract

The nature of the phonon triplet in the region of OH-stretching modes in hydrogen-bonded materials is often explained by the interplay of OH-stretching modes and combinations and overtones of OH-bending modes. In order to elucidate the both contributions in KH$_2$PO$_4$ (KDP), we compare the pressure dependence of the OH-bending and stretching modes from ab initio calculation and inelastic neutron scattering (INS) measurements. The ab initio calculation predicts a hardening of OH-bending modes and a softening of OH-stretching modes with pressure. At the same time, INS measurements in the region of OH-stretching modes indicate a hardening of the phonon triplet together with the bending modes. This means that this triplet in INS measurements is mainly due to combinations and overtones of OH-bending modes, while the intensity of OH-stretching modes appears to be relatively low. This conclusion may also apply to other hydrogen-bonded materials.