Multiple Twinning As a Structure Directing Mechanism in Layered Rock-Salt-Type Oxides: NaMnO₂ Polymorphism, Redox Potentials, and Magnetism

New polymorphs of NaMnO 2 have been observed using transmission electron microscopy and synchrotron X-ray powder diffraction. Coherent twin planes confined to the (NaMnO 2 ) layers, parallel to the (101̅) crystallographic planes of the monoclinic layered rock-salt-type α-NaMnO 2 (O3) structure, form quasi-periodic modulated sequences, with the known α- and β-NaMnO 2 polymorphs as the two limiting cases. The energy difference between the polymorphic forms, estimated using a DFT-based structure relaxation, is on the scale of the typical thermal energies that results in a high degree of stacking disorder in these compounds. The results unveil the remarkable effect of the twin planes on both the magnetic and electrochemical properties. The polymorphism drives the magnetic ground state from a quasi-1D spin system for the geometrically frustrated α-polymorph through a two-leg spin ladder for the intermediate stacking sequence toward a quasi-2D magnet for the β-polymorph. A substantial increase of the equilibrium potential for Na deintercalation upon increasing the concentration of the twin planes is calculated, providing a possibility to tune the electrochemical potential of the layered rock-salt ABO 2 cathodes by engineering the materials with a controlled concentration of twins.