Multiple Twinning As a Structure Directing Mechanism
in Layered Rock-Salt-Type Oxides: NaMnO<sub>2</sub> Polymorphism,
Redox Potentials, and Magnetism
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Abstract
New polymorphs of NaMnO<sub>2</sub> have been observed using transmission
electron microscopy and synchrotron X-ray powder diffraction. Coherent
twin planes confined to the (NaMnO<sub>2</sub>) layers, parallel to
the (101̅) crystallographic planes of the monoclinic layered
rock-salt-type α-NaMnO<sub>2</sub> (O3) structure, form quasi-periodic
modulated sequences, with the known α- and β-NaMnO<sub>2</sub> 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<sub>2</sub> cathodes by engineering the materials
with a controlled concentration of twins