Challenges in the fields of renewable energy harvesting, data storage, and nanoelectronics have resparked interest in ferroelectric domain walls (DWs) as tunable, nanoscale elements. However, the study of such structures has mostly relied on 2-dimensional, rather slow imaging techniques such as scanning probe microscopy. Therefore, Cherenkov second harmonic generation (CSHG) microscopy has been established as a technique suitable for the nondestructive imaging of ferroelectric DWs and their 3dimensional (3D) evolution. Here, we report on the real-time and in situ 3D DW kinetics when inspecting electrical-fieldbiased 200 μm thick lithium niobate (LNO) single crystals. A linear electric field increase up to +4.0 kV/mm (antiparallel to the direction of spontaneous polarization P S ) resulted in the collapse of laser-poled hexagonal domains into cone-like structures. The average inclination was measured to rise up to 2.5°. Head-to-head (h2h) domain walls dominated. Simultaneously, the domain wall current (DWC) was recorded in situ. It increased by 4 orders of magnitude to 1 μA. The DW mobility increased dramatically as a function of depth. Moreover, a significant asymmetry was found, as DW mobility was much higher along crystallographic (Y+) directions. Subsequently, the electric field was reversed and swept to −3.6 kV/mm. While the hexagonal domain shapes were restored for moderate electric fields, the domains separated into many nucleating spike domains when exceeding a critical threshold of −3.5 kV/mm. DWC increased dramatically through this process, reaching magnitudes of up to 1 mA. The understanding of DW dynamics upon electric stimulation was used to realize a two-port DW-based nanoswitch. Alternating positive and negative electric fields were applied to a neutral hexagonal domain contacted purely via solid electrodes. The field strengths were kept well below the critical threshold of spike domain formation. The electrical conductivity of such a device could be tuned over 4 orders of magnitude, i.e., deliberately switched on and off. Our findings support the development of future DW-based nanoelectronic devices.

Orange-colored crystals of the oxoferrate tellurate K 12 + 6x Fe 6 Te 4À x O 27 [x = 0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water-base ratio n(H 2 O)/n(KOH) of 1.5 starting from Fe(NO 3 ) 3 • 9H 2 O, TeO 2 and H 2 O 2 at about 200 °C. By using (NH 4 ) 2 TeO 4 instead of TeO 2 , a fine powder consisting of microcrystalline spheres of K 12 + 6x Fe 6 Te 4À x O 27 was obtained. K 12 + 6x Fe 6 Te 4À x O 27 crystallizes in the acentric cubic space group I4 ̄3d. [Fe III O 5 ] pyramids share their apical atoms in [Fe 2 O 9 ] groups and two of their edges with [Te VI O 6 ] octahedra to form an open framework that consists of two loosely connected, but not interpenetrating, chiral networks. The flexibility of the hinged oxometalate network manifests in a piezoelectric response similar to that of LiNbO 3 .The potassium cations are mobile in channels that run along the < 111 > directions and cross in cavities acting as nodes. The ion conductivity of cold-pressed pellets of ballmilled K 12 + 6x Fe 6 Te 4À x O 27 is 2.3 × 10 À 4 S • cm À 1 at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe 2 O 9 ] groups.

Ferroelectric domain boundaries are quasi-two-dimensional functional interfaces with high prospects for nanoelectronic applications. Despite their reduced dimensionality, they can exhibit complex non-Ising polarization configurations and unexpected physical properties. Here, the impact of the three-dimensional (3D) curvature on the polarization profile of nominally uncharged 180° domain walls in LiNbO3 is studied using second-harmonic generation microscopy and 3D polarimetry analysis. Correlations between the domain-wall curvature and the variation of its internal polarization unfold in the form of modulations of the Néel-like character, which we attribute to the flexoelectric effect. While the Néel-like character originates mainly from the tilting of the domain wall, the internal polarization adjusts its orientation due to the synergetic upshot of dipolar and monopolar bound charges and their variation with the 3D curvature. Our results show that curved interfaces in solid crystals may offer a rich playground for tailoring nanoscale polar states.

Strongly charged head-to-head domain walls that are purposely engineered along the [110] crystallographic orientation into ferroelectric BaTiO3 single crystals have been proposed as intrinsically nanoscaled two-dimensional electron gases (2DEGs) because of their significant conductivity. Here, we quantify these 2DEG properties through dedicated Hall transport measurements in van der Pauw 4-point geometry, finding the electron mobility to reach around 400 cm2 (V s)−1, while the two-dimensional charge density amounts to 7 × 103 cm–2. We underline the necessity to take into account the thermal and geometrical misalignment offset voltages by evaluating the Hall resistance under magnetic field sweeps; otherwise, errors of several hundred percent in the derived transport parameters can occur.

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