Spontaneous Compositional Nanopatterning in Li-Containing Perovskite Oxides

Coupled Compositional and Displacive Modulations in KLaMnWO6 Revealed by Atomic Resolution Imaging

Complex compositional and displacive modulations of the crystal structure of KLaMnWO₆ are imaged with atomic resolution by means of scanning transmission electron microscopy (STEM). This oxide is stabilized by cation vacancies leading to a La_1+x/3K_1-xMnWO₆ stoichiometry. Compositional modulation on both the K and La layers are revealed in the high-angle annular dark-field STEM (HAADF-STEM) images. The compositional modulation within the La layer is coupled with the modulation of the octahedral tilting, which is exposed by imaging of the anion sublattice in annular bright-field STEM (ABF-STEM) images. These complex modulations are accommodated in a 5√2a_p × 5√2a_p × 2a_p perovskite-type structure.

Lithography-Free Electrochemical Patterning of Conductive Substrates with Metal Oxides

Reactive interface patterning promoted by lithographic electrochemistry serves as a facile method for generating submicron structures on conductive substrates. A binary-potential step applied to a metal layer with a resist overlayer allows silicon to be patterned with metal oxides. In this study, the role and influence of the resist overlayer on the uniformity of pattern formation are examined. The ability of the resist to detach from the underlying metal is a critical determinant of pattern geometry. By choosing an appropriate resist, large patterns with submicron precision are generated quickly by the application of the binary-potential steps. From this information, a lithography-free approach to generating identical patterns is achieved with simple resists such as that furnished from a lacquer-water emulsion, thus greatly simplifying the patterning of silicon with metal oxide catalysts.

Direct mapping of Li-enabled octahedral tilt ordering and associated strain in nanostructured perovskites

Self-assembled nanostructures with periodic phase separation hold great promise for creating two- and three-dimensional superlattices with extraordinary physical properties. Understanding the mechanism(s) driving the formation of such superlattices demands an understanding of their underlying atomic structure. However, the nanoscale structural fluctuations intrinsic to these superlattices pose a new challenge for structure determination methods. Here we develop an optimized atomic-level imaging condition to measure TiO6 octahedral tilt angles, unit-cell-by-unit-cell, in perovskite-based Li(0.5-3x)Nd(0.5+x)TiO3, and thereby determine the mathematical formula governing this nanoscale superstructure. We obtain a direct real-space correlation of the octahedral tilt modulation with the superstructure geometry and lattice-parameter variations. This reveals a composition-dependent, self-ordered octahedral superlattice. Amazingly, we observe a reversible annihilation/reconstruction of the octahedral superlattice correlated with the delithiation/lithiation process in this promising Li-ion conductor. This approach to quantify local octahedral tilt and correlate it with strain can be applied to characterize complex octahedral behaviours in other advanced oxide systems.

Frenkel-Defect-Mediated Chemical Ordering Transition in a Li-Mn-Ni Spinel Oxide

Using spinel-type Li(Mn(1.5)Ni(0.5) )O4 with two different cations, Mn and Ni, in the oxygen octahedra as a model system, we show that a cation ordering transition takes place through the formation of Frenkel-type point defects. A series of experimental results based on atomic-scale observations and in situ powder diffractions along with ab initio calculations consistently support such defect-mediated transition behavior. In addition to providing a precise suggestion of the intermediate transient states and the resulting kinetic pathway during the transition between two phases, our findings emphasize the significant role of point defects in ordering transformation of complex oxides.