We present a detailed high pressure investigation on the magnetoelectric material Co₄Nb₂O₉ by means of Raman spectroscopy, synchrotron x-ray diffraction, and dielectric constant measurements combined with the density functional theory (DFT) based simulations. Our experimental results demonstrate three isostructural phase transitions at about 5.2 (P1), 8.5 (P2), and 11.5 GPa (P3) as evidenced by the distinct slope change in lattice parameters along with the splitting of certain Raman modes. Profound anomalies have been observed in linewidth and mode shift of several Raman modes as a consequence of isostructural transitions in the vicinity of P1, P2, and P3. Low temperature Raman spectroscopic investigation shows that strong spin-phonon coupling is present in the system up to 225 K due to short-range magnetic interactions. The onset of two crystal symmetry transitions at around 17 (P4) and 21.5 (P5) is manifested by the appearance of new Raman modes and the emergence/splitting of several Bragg peaks, which are corroborated by DFT calculations through enthalpy crossover. The system exhibits a mixed phase of P-3c1 and monoclinic C2/c in the pressure range of 17-20 GPa. At P5, the P-3c1 phase is replaced by a more distorted monoclinic P2₁/c phase, which coexists with the C2/c phase up to 24 GPa.
As per the DFT insight, the huge charge loss by the O atom in the buckled honeycomb layer around the pressure range 9–13 GPa may attributes to the electronic origin of structural transition.