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. To elucidate both contributions in ${\mathrm{KH}}_2{\mathrm{PO}}_4$ (KDP), we compare the pressure dependence of the OH-bending and stretching modes from ab initio calculations and inelastic neutron scattering (INS) measurements. The ab initio calculations predict 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.

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• Anharmonic lattice dynamics
• High-pressure studies
• Dielectrics
• Ferroelectrics
• Density functional calculations
• Inelastic neutron scattering