Measuring Electric Field at Interfaces of Fe2O3 and Cr2O3 via High Resolution S/TEM Methods

Structural materials in nuclear reactors are exposed to coupled extremes such as irradiation, high temperatures, and corrosive environments, all of which can drastically modify their oxide layers and degrade performance. One of the major challenges for advanced reactors is ensuring that protective oxide coatings sustain their structural and chemical integrity under these conditions. The local electric field (E-field) critically governs ion transport through the oxide and defect formation [1, 2]. Therefore, insights into interface stability, phase evolution, and design strategies for optimizing reactor component materials can stem from understanding how point and extended defects (voids, bubbles, black spot damage), introduced by irradiation and elevated temperature, alter the E-field across oxide interfaces. This study implements scanning transmission electron microscopy techniques including HAADF-STEM, STEM-EELS and 4D-STEM differential phase contrast (DPC) imaging to examine model 2-layer Fe2O3–Cr2O3 oxide (Fig. 1a) and 3-layer Cr2O3–Fe2O3–Cr2O3 oxide thin films exposed to 100 keV Fe-ion irradiation. Specifically, we quantify local changes in the E-field across oxide interfaces at nanoscale spatial resolution by using a combination of EELS mapping (Fig. 1b) to measure sample thickness [3] and DPC mapping to measure the center of mass (CoM) of the transmitted diffraction disc. The deflection of the electron beam, which experiences subtle shifts in momentum (i.e., CoM shift), is proportional to the local E-field, resulting in contrast variations that can be converted into E-field maps (Fig. 1c,d) [4, 5]. By comparing samples with and without Fe-ion (0.1 dpa) irradiation, we can discern how charged defect generation, charge redistribution, and structural disorder alter the E-field landscape near the Fe2O3–Cr2O3 interface. A sharp drop observed in the E-field line profiles (Fig. 1e-f) is associated with the difference in anion charge distribution across the interface. The unirradiated condition in the out-of-plane direction (along ⟨0001⟩⁠) displays a measured E-field of ∼2.53×107 V/m, while the irradiated condition exhibits a reduction to ∼1.84×106 V/m. By integrating the measured E-field, we estimate the electrostatic potential (Fig. 1g,h), which shows a shift from ∼0.25 V to ∼0.08 V and is compared to the valence band offset simulated by density functional theory (DFT). With guidance from DFT simulations of the Fe2O3–Cr2O3 system [6], subtle variations in oxygen anion intermixing and dipole distributions can be correlated with the measured E-field profiles, thereby revealing interfacial conduction properties and the change in the potential barrier after irradiation. The findings from this work highlight the significance of the unique conditions that alter the evolution of charged defects and the electronic properties of oxide thin films under irradiated nuclear reactor environments [7]. Electric field of oxide interfaces with respect to irradiation. (a) HAADF image of unirradiated 2-layer oxide; (b) absolute thickness map of 2-layer oxide samples. DPC E-field map of (c) no irradiation sample and (d) irradiated sample; E-field line profile across the interface (91 pixels width averaged) of (e) no irradiation sample and (f) irradiated sample; and potential along out-of-plane direction (integral with reference 0 point at Fe2O3 surface): (g) no irradiation sample and (h) irradiated sample. Note: the regime between the two black dashed lines indicates where the 2-layer oxide interface is located and is used as a reference for the potential curves. (Scale bars = 100 nm).