  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 $Fe_4Nb_2O_9$.
