Search for: All records

Na_xMnO₂shows Mn³⁺and Mn⁴⁺charge separation with the charge stripe ordering upon Na deintercalation atx= 5/8. In this paper it is shown that, surprisingly, at lower Na compositions of 5/8 >x≥ 1/18 the phase evolution pathway of Na_xMnO₂upon Na deintercalation shows a unique phenomenon of super charge separation, where the Mn³⁺and Mn⁴⁺ions fully charge‐separate into charge superplanes formed by succession of charge stripes in the third dimension. The Mn³⁺superplanes attract Na ions electronically, and dominate the antiferromagnetic interactions in NaMnO₂. Na ions in Mn³⁺superplanes also naturally pillar the MnO₂layers to form the unusual O1 phases with large interlayer distances atx< 1/3, which dominates the unique electrochemical behavior of NaMnO₂.

Since 2011, ferroelectric HfO₂has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake‐up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO₂to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake‐up and fatigue phenomena. The wake‐up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO₂layer near the electrodes. The evolution of these aspects of structuring contributes to the increase inP_rand the opening of the constrictedP–Vhysteresis that are known to occur with wake‐up. The onset of the fatigue regime is correlated to an increasing concentration of bulk defects, which are proposed to pin domain walls.

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high‐endurance memory applications like nonvolatile random‐access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field‐induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake‐up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance.