The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, ${\mathrm{Mn}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ (MNO) and ${\mathrm{Mn}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ (MTO), has been investigated using Raman spectroscopy, synchrotron x-ray diffraction, and density functional theory (DFT) calculations. In MTO, application of a small pressure of only ∼0.5 GPa induces an isostructural transition driven by local symmetry breaking. With further increase in pressure, three additional isostructural transitions are observed, around 3.2, 6, and 10 GPa, followed by the onset of a long-range structural transition near 14 GPa, where the ambient $P\text{−}3c1$ phase begins to transform to a $P2/c$ phase. These two phases coexist up to 27 GPa. The Nb analog, MNO, also exhibits similar isostructural transitions at approximately 2, 6.6, and 10 GPa; however, the onset of the mixed $P2/c$ and $P\text{−}3c1$ phases occurs at a slightly lower pressure ($\sim 12.5\mathrm{GPa}$), with coexistence extending up to 26.5 GPa. These long-range transitions are supported by pressure-dependent enthalpy variations derived from DFT computations. Rietveld refinement reveals pronounced anisotropic lattice compression ($\sim 42–49\%$) between the $c$ and $a$ axes, leading to a notable reduction in the $c/a$ ratio. This anisotropy may enhance interlayer coupling under pressure and could potentially influence magnetic interactions, as suggested by the emergence of Raman modes resembling those reported at low temperatures, along with anomalous variations in Raman mode linewidth and intensity, warranting further direct investigations of the magnetic response under compression. The pronounced changes in Raman self-energy parameters, anomalies in the reduced pressure–Eulerian strain profile, and the onset of local symmetry breaking at much lower pressures in MTO than in MNO highlight the crucial role of differing spin-orbit coupling strength and orbital hybridization effects associated with ${\mathrm{Nb}}^{5+}$ and ${\mathrm{Ta}}^{5+}$ cations.

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• Crystal structure
• Phase transitions
• Pressure effects
• Spin-phonon coupling
• Multiferroics
• Strongly correlated systems
• Density functional theory
• Raman spectroscopy
• X-ray diffraction