Multiple Twinning As a Structure Directing Mechanism in Layered Rock-Salt-Type Oxides: NaMnO 2 Polymorphism, Redox Potentials, and Magnetism Artem M. Abakumov,* ,† Alexander A. Tsirlin, ‡ Ioanna Bakaimi, §,∥ Gustaaf Van Tendeloo, † and Alexandros Lappas § † EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Antwerp, Belgium ‡ National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia § Institute of Electronic Structure and Laser, Foundation for Research and Technology - Hellas, Vassilika Vouton, 71110 Heraklion, Greece ∥ Department of Physics, University of Crete, Voutes, 71003 Heraklion, Greece * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION The ABO 2 (A = alkali metal, B = transition metal) complex oxides are arguably the most popular family of compounds used as cathode materials in rechargeable batteries. Some of them, such as LiCoO 2 , have already been commercialized in today’s portable electronic devices. 1 Others, such as Li- Ni 1/3 Co 1/3 Mn 1/3 O 2 , are intensively investigated because of their high reversible capacities (≈200 mAh/g). 2 The crystal structures of these complex oxides can be considered as derivatives of the rock-salt structure based on the cubic (ABCABC or ccc) close packing of the oxygen atoms, where all octahedral interstices are filled with the A and B cations. 3 The charge and size difference between the A and B cations is the driving force behind cation ordering. In the layered ABO 2 structure, the A and B cations are arranged into separate layers parallel to the {111} close-packed plane of the parent rock-salt structure. Such an ordered structure, also referred to as the α- NaFeO 2 type or the O3 structure, adopts a hexagonal unit cell with the R3̅m symmetry, a h = 1/2(a RS − b RS ), b h = 1/2(b RS − c RS ), and c h = 2(a RS + b RS + c RS ), where RS stands for the rock- salt arystotype. The layered cation ordering results in facile diffusion paths for the alkali A cations, thus ensuring their fast mobility in these structures. 4 Similar to O 3 -LiCoO 2 , the α-NaMnO 2 phase features a layered structure, which is suitable for the reversible electro- chemical Na removal/insertion (Figure 1a). About 0.85 Na can be deintercalated from α-NaMnO 2 , and 0.8 Na can be intercalated back demonstrating a capacity of ∼132 mAh/g after 20 cycles. 5 The α-NaMnO 2 phase is a distorted variant of the α-NaFeO 2 structure due to a significant deformation of the MnO 6 octahedra caused by the Jahn−Teller effect, which is inherent to the high-spin Mn 3+ cations. An apical elongation of the MnO 6 octahedra with the formation of two long (∼2.40 Å) and four short (∼1.93 Å) Mn − O separations decreases the symmetry down to monoclinic C2/m with the unit cell vectors related to the basis vectors of the R3̅m α-NaFeO 2 structure as a m =2a h + b h , b m = b h , c m = 1/3(−2a h − b h + c h ). 6 α-NaMnO 2 is used as a precursor for the layered form of LiMnO 2 , a widely Received: April 2, 2014 Revised: April 30, 2014 Published: May 1, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 3306 dx.doi.org/10.1021/cm5011696 | Chem. Mater. 2014, 26, 3306−3315