Crystal and magnetic structures of the high-pressure-stabilized perovskite modification of Mn2O3 (ζ -Mn2O3) have been studied by neutron powder diffraction combined with symmetry arguments based on the phenomenological Landau theory. This metastable phase exhibits a unique charge disproportionation phenomenon stabilizing the quadruple perovskite structure (Mn2+Mn3+ 3 )Mn3.25+ 4 O12 with an additional charge-ordering and commensurate orbital density wave localized in the B-site perovskite position. The commensurate nature of the orbital density wave is stimulated by a coupling of the orbital ordering to independent structural distortions, which improve poor bonding conditions of Mn2+ in the A-site perovskite position. Below T1 ∼ 100 K, an anharmonic longitudinal spin density wave arises and locks to the structural modulation associated with the orbital density. At T2 ∼ 50 K, the magnetic subsystem delocks from the structural modulation giving rise to a multi-k phase-modulated ground state admixing cycloidal and helical components. The complex anharmonic and phase-modulated magnetic structures are discussed based on a phenomenological magneto-orbital coupling scheme, previously developed to explain the multi-k helical ground states with modulated spin chirality observed in A2+Mn7O12 (A2+ = Ca, Sr, Pb, and Cd) quadruple perovskite
