Pressure driven re-entrant magnetoelectric transition in honeycomb $Fe_{4}Nb_{2}O_{9}$

TL;DR

This study reveals a pressure-induced structural and magnetic phase transition in Fe4Nb2O9, accompanied by a re-entrant magnetoelectric effect, without an insulator-metal transition, as shown by x-ray and Raman spectroscopy analyses.

Contribution

It provides the first detailed high-pressure analysis of Fe4Nb2O9, demonstrating a structural transition and re-entrant magnetoelectric behavior driven by pressure-induced distortions.

Findings

Structural transition from trigonal to monoclinic above 8.8 GPa.

Pressure enhances ferroelectric polarization up to 8.8 GPa.

No insulator-metal transition observed under pressure.

Abstract

A detailed high pressure investigation is carried out on $Fe_4Nb_2O_9$ using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of $Nb-O_6$ octahedra as seen from x-ray diffraction analysis and a large pressure dependence of $Nb-O_6$ octahedra breathing Raman mode. Anomalous behaviour of Raman modes and increase in the phonon life time at the phase transition pressure indicate a trigonal paramagnetic phase to a monoclinic antiferromagnetic state above 8.8 GPa. Decrease in the diffusive scattering rate of low frequency electronic contribution contradicts the results of decrease in intensity of high frequency electronic response and excludes the phenomenon of insulator to metal transition. Instead the enhancement of the intensity of the Raman modes till about 8.8 GPa indicate a large change in ferroelectric polarization of the sample indicating pressure induced re-entrant magentoelectric effect in $Fe_4Nb_2O_9$.