Raman Spectroscopic Investigation of Ferroaxial Order in Na2BaNi(PO4)2 Single Crystals

TL;DR

This study uses polarization-resolved Raman spectroscopy and first-principles calculations to identify phonons linked to ferroaxial order in Na2BaNi(PO4)2, providing a symmetry-based framework for analyzing rotational distortions.

Contribution

It offers the first symmetry-consistent identification of Raman-active phonons associated with ferroaxial order in a complex oxide, combining experimental and computational methods.

Findings

Identification of specific low-frequency Ag modes with linewidth broadening

Establishment of a symmetry-guided spectroscopic framework

Insights into phonon behavior related to rotational distortions

Abstract

Ferroaxial order is characterized by the breaking of mirror symmetry parallel to the crystallographic principal axis, which often originates from spontaneous rotational distortions of the crystal lattice. Such rotational distortions are, by symmetry, allowed to couple to specific phonon modes. However, Raman-active phonons associated with these rotational distortions have not yet been clearly identified on a symmetry-consistent basis. Here, we perform polarization-resolved Raman spectroscopy on the ferroaxial phase of Na2BaNi(PO4)2 single crystals and combine the measurements with first-principles lattice-dynamics calculations. This symmetry-guided analysis enables a comprehensive assignment of Raman-active modes in the ferroaxial phase. Several low-frequency Ag modes exhibit finite linewidth broadening, suggesting that these phonons may be weakly affected by the underlying rotational distortion. These results establish a symmetry-based spectroscopic framework for analyzing phonons associated with rotational distortions in ferroaxial materials and provide a basis for future studies of ferroaxial order in complex oxides.