Pressure driven phase transitions in honeycomb Fe4Nb2O9: A possible re-entrant multiferroic behavior.

A detailed high-pressure investigation is carried out on F e 4 N b 2 O 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 an Nb – O 6 octahedra breathing Raman mode. Anomalous behavior of Raman modes and an increase in the phonon lifetime at the phase transition pressure indicate a possible change in the magnetic property of the sample above 8.8 GPa. A decrease in the diffusive scattering rate of a low-frequency electronic contribution contradicts the results of a decrease in the intensity of a high-frequency electronic response and excludes the phenomenon of an insulator to metal transition. On the contrary, the enhancement of the intensity of the Raman modes up to about 8.8 GPa indicates a large change in ferroelectric polarization of the sample, indicating a possible pressure induced re-entrant multiferroic behavior in F e 4 N b 2 O 9 . [ABSTRACT FROM AUTHOR]