The Electrodegradation Process in PZT Ceramics under Exposure to Cosmic Environmental Conditions.

Long-time electric field action on perovskite piezoelectric ceramic leads to chemical degradation. A new way to accelerate the degradation is the exposure of the ceramic to DC electric fields under a vacuum. A high-quality commercial piezoelectric material based on PbZr1-xTixO3 is used to study such impacts. To avoid the influence of ferroelectric properties and possible removal of oxygen and lead oxides during the degradation process, the experiments are in the temperature interval of 500 °C > T > TC. Changes in resistance during the electrodegradation process is an electrically-induced deoxidation, transforming the ceramic into a metallic-like material. This occurs with an extremely low concentration of effused oxygen of 1016 oxygen atoms per 1 cm3. Due to this concentration not obeying the Mott criterion for an isolator-metal transition, it is stated that the removal of oxygen mostly occurs along the grain boundaries. It agrees with the first-principle calculations regarding dislocations with oxygen vacancies. The decrease in resistivity during electrodegradation follows a power law and is associated with a decrease in the dislocation dimension. The observed reoxidation process is a lifeline for the reconstructing (self-healing) properties of electro-degraded ceramics in harsh cosmic conditions. Based on all of these investigations, a macroscopic and nanoscopic model of the electrodegradation is presented.

Atomistic level aqueous dissolution dynamics of NASICON-Type Li1+xAlxTi2-x(PO4)3 (LATP).

Advancing the atomistic level understanding of aqueous dissolution of multicomponent materials is essential. We combined ReaxFF and experiments to investigate the dissolution at the Li1+xAlxTi2-x(PO4)3-water interface. We demonstrate that surface dissolution is a sequentially dynamic process. The phosphate dissolution destabilizes the NASICON structure, which triggers a titanium-rich secondary phase formation.

Dynamics of the Chemically Driven Densification of Barium Titanate Using Molten Hydroxides.

Molten hydroxides, often used for crystal growth and nanoparticle synthesis, have recently been applied for the single step densification of several inorganic materials under moderate uniaxial pressures and 1000 °C below their usual sintering temperatures. The latter approach, termed cold sintering process (CSP), is a mechanochemically driven process that enables the densification of inorganic materials through a dissolution-precipitation creep mechanism. In this study, we report the main densification mechanisms of BaTiO3 in a NaOH-KOH eutectic mixture. A chemical insight at the atomistic level, investigated by ReaxFF molecular dynamics simulations, offers plausible ionic complex formation scenarios and reactions at the BaTiO3/molten hydroxide interface, enabling the dissolution-precipitation reactions and the subsequent cold sintering of BaTiO3.

Assessment of the Role of Speciation during Cold Sintering of ZnO Using Chelates.

Cold sintering (CS) is a chemically driven densification technique enabling a substantial decrease in the sintering temperature of oxides, by several hundreds of degrees Celsius. Although the densification process in CS is known to be mainly driven by pressure solution creep, additional fundamental aspects driving the interfacial chemistry reactions are still a subject of debate. Herein, we focus on the aspect of speciation in the densification process. The densification of zinc oxide (ZnO) by CS using zinc acetylacetonate hydrate (Zn(acac)2·xH2O), a versatile ligand often used as a precursor for ZnO synthesis in wet chemistry, is reported. The successful densification of ZnO using H2O and Zn(acac)2·xH2O confirms the importance of speciation in CS, as ZnO has a very low solubility in pure H2O. The evolution of the system at different stages of sintering and the role of the Zn(acac)2·xH2O species were evaluated.

Water-Mediated Surface Diffusion Mechanism Enables the Cold Sintering Process: A Combined Computational and Experimental Study.

The cold sintering process (CSP) densifies ceramics at much lower temperatures than conventional sintering processes. Several ceramics and composite systems have been successfully densified under cold sintering. For the grain growth kinetics of zinc oxide, reduced activation energies are shown, and yet the mechanism behind this growth is unknown. Herein, we investigate these mechanisms in more detail with experiments and ReaxFF molecular dynamics simulations. We investigated the recrystallization of zinc cations under various acidic conditions and found that their adsorption to the surface can be a rate-limiting factor for cold sintering. Our studies show that surface hydroxylation in CSP does not inhibit crystallization; in contrast, by creating a surface complex, it creates an orders of magnitude acceleration in surface diffusion, and in turn, accelerates recrystallization.

Development of a ReaxFF reactive force field for lithium ion conducting solid electrolyte Li1+xAlxTi2-x(PO4)3 (LATP).

We developed a ReaxFF reactive force field for NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP) materials, which is a promising solid-electrolyte that may enable all-solid-state lithium-ion batteries. The force field parameters were optimized based on density functional theory (DFT) data, including equations of state and the heats of formation of ternary metal oxides and metal phosphate crystal phases (e.g., LixTiO2, Al2TiO5, LiAlO2, AlPO4, Li3PO4 and LiTi2(PO4)3 (LTP)), and the energy barriers for Li diffusion in TiO2 and LTP via vacancies and interstitial sites. Using ReaxFF, the structural and the energetic features of LATP were described properly across various compositions - Li occupies more preferentially the interstitial site next to Al than next to Ti. Also, as observed in experimental data, the lattice parameters decrease when Ti is partly substituted by Al because of the smaller size of the Al cation. Using this force field, the diffusion mechanism and the ionic conductivity of Li in LTP and LATP were investigated at T = 300-1100 K. Low ionic conductivity (5.9 × 10-5 S cm-1 at 300 K) was obtained in LTP as previously reported. In LATP at x = 0.2, the ionic conductivity was slightly improved (8.4 × 10-5 S cm-1), but it is still below the experimental value, which is on the order of 10-4 to 10-3 S cm-1 at x = 0.3-0.5. At higher x (higher Al composition), LATP has a configurational diversity due to the Al substitution and the concomitant insertion of Li. By performing a hybrid MC/MD simulation for LATP at x = 0.5, a thermodynamically stable LATP configuration was obtained. The ionic conductivity of this LATP configuration was calculated to be 7.4 × 10-4 S cm-1 at 300 K, which is one order of magnitude higher than the ionic conductivity for LTP and LATP at x = 0.2. This value is in good agreement with our experimental value (2.5 × 10-4 S cm-1 at 300 K) and the literature values. The composition-dependent ionic conductivity of LATP was successfully demonstrated using the ReaxFF reactive force field, verifying the applicability of the LATP force field for the understanding of Li diffusion and the design of highly Li ion conductive solid electrolytes. Furthermore, our results also demonstrate the feasibility of the MC/MD method in modeling LATP configuration, and provide compelling evidence for the solid solution sensitivity on ionic conductivity.

ReaxFF molecular dynamics simulation of intermolecular structure formation in acetic acid-water mixtures at elevated temperatures and pressures.

The intermolecular structure formation in liquid and supercritical acetic acid-water mixtures was investigated using ReaxFF-based molecular dynamics simulations. The microscopic structures of acetic acid-water mixtures with different acetic acid mole fractions (1.0 ≥ xHAc ≥ 0.2) at ambient and critical conditions were examined. The potential energy surface associated with the dissociation of acetic acid molecules was calculated using a metadynamics procedure to optimize the dissociation energy of ReaxFF potential. At ambient conditions, depending on the acetic acid concentration, either acetic acid clusters or water clusters are dominant in the liquid mixture. When acetic acid is dominant (0.4 ≤ xHAc), cyclic dimers and chain structures between acetic acid molecules are present in the mixture. Both structures disappear at increased water content of the mixture. It was found by simulations that the acetic acid molecules released from these dimer and chain structures tend to stay in a dipole-dipole interaction. These structural changes are in agreement with the experimental results. When switched to critical conditions, the long-range interactions (e.g., second or fourth neighbor) disappear and the water-water and acetic acid-acetic acid structural formations become disordered. The simulated radial distribution function for water-water interactions is in agreement with experimental and computational studies. The first neighbor interactions between acetic acid and water molecules are preserved at relatively lower temperatures of the critical region. As higher temperatures are reached in the critical region, these interactions were observed to weaken. These simulations indicate that ReaxFF molecular dynamics simulations are an appropriate tool for studying supercritical water/organic acid mixtures.

Dielectric relaxation and localized electron hopping in colossal dielectric (Nb,In)-doped TiO2 rutile nanoceramics.

Dielectric spectroscopy was performed on a Nb and In co-doped rutile TiO2 nano-crystalline ceramic (n-NITO) synthesized by a low-temperature spark plasma sintering (SPS) technique. The dielectric properties of the n-NITO were not largely affected by the metal electrode contacts. Huge dielectric relaxation was observed at a very low temperature below 35 K. Both the activation energy and relaxation time suggested that the electronic hopping motion is the underlying mechanism responsible for the colossal dielectric permittivity (CP) and its relaxation, instead of the internal barrier layer effect or a dipolar relaxation. With Havriliak-Negami (H-N) fitting, a relaxation time with a large distribution of dielectric relaxations was revealed. The broad distributed relaxation phenomena indicated that Nb and In were involved, controlling the dielectric relaxation by modifying the polarization mechanism and localized states. The associated distribution function is calculated and presented. The frequency-dependent a.c. conductance is successfully explained by a hopping conduction model of the localized electrons with the distribution function. It is demonstrated that the dielectric relaxation is strongly correlated with the hopping electrons in the localized states. The CP in SPS n-NITO is then ascribed to a hopping polarization.

Study on the behavior of atomic layer deposition coatings on a nickel substrate at high temperature.

Although many techniques have been applied to protect nickel (Ni) alloys from oxidation at intermediate and high temperatures, the potential of atomic layer deposition (ALD) coatings has not been fully explored. In this paper, the application of ALD coatings (HfO2, Al2O3, SnO2, and ZnO) on Ni foils has been evaluated by electrical characterization and transmission electron microscopy analyses in order to assess their merit to increase Ni oxidation resistance; particular consideration was given to preserving Ni electrical conductivity at high temperatures. The results suggested that as long as the temperature was below 850 °C, the ALD coatings provided a physical barrier between outside oxygen and Ni metal and hindered the oxygen diffusion. It was illustrated that the barrier power of ALD coatings depends on their robustness, thicknesses, and heating rate. Among the tested ALD coatings, Al2O3 showed the maximum protection below 900 °C. However, above that temperature, the ALD coatings dissolved in the Ni substrate. As a result, they could not offer any physical barrier. The dissolution of ALD coatings doped on the NiO film, formed on the top of the Ni foils. As found by the electron energy loss spectroscopy (EELS), this doping affected the electronic transport process, through manipulating the Ni(3+)/Ni(2+) ratio in the NiO films and the chance of polaron hopping. It was demonstrated that by using the ZnO coating, one would be able to decrease the electrical resistance of Ni foils by two orders of magnitude after exposure to 1020 °C for 4 min. In contrast, the Al2O3 coating increased the resistance of the uncoated foil by one order of magnitude, mainly due to the decrease in the ratio of Ni(3+)/Ni(2+).

Cold Sintering: A Paradigm Shift for Processing and Integration of Ceramics.

This paper describes a sintering technique for ceramics and ceramic-based composites, using water as a transient solvent to effect densification (i.e. sintering) at temperatures between room temperature and 200 °C. To emphasize the incredible reduction in sintering temperature relative to conventional thermal sintering this new approach is named the "Cold Sintering Process" (CSP). Basically CSP uses a transient aqueous environment to effect densification by a mediated dissolution-precipitation process. CSP of NaCl, alkali molybdates and V2 O5 with small concentrations of water are described in detail, but the process is extended and demonstrated for a diverse range of chemistries (oxides, carbonates, bromides, fluorides, chlorides and phosphates), multiple crystal structures, and multimaterial applications. Furthermore, the properties of selected CSP samples are demonstrated to be essentially equivalent as samples made by conventional thermal sintering.

Influence of Ce substitution for Bi in BiVO4 and the impact on the phase evolution and microwave dielectric properties.

In the present work, the (Bi1-xCex)VO4 (x ≤ 0.6) ceramics were prepared via a solid-state reaction method and all the ceramic samples could be densified below 900 °C. From the X-ray diffraction analysis, it is found that a monoclinic scheelite solid solution can be formed in the range x ≤ 0.10. In the range 0.20 ≤ x ≤ 0.60, a composite region with both monoclinic scheelite and tetragonal zircon solid solutions was formed and the content of the zircon phase increased with the calcined or sintering temperature. The refined lattice parameters of (Bi0.9Ce0.1)VO4 are a = 5.1801(0) Å, b = 5.0992(1) Å, c = 11.6997(8) Å, and γ = 90.346(0)° with the space group I112/b(15). The VO4 tetrahedron contracts with the substitution of Ce for Bi at the A site, and this helps to keep the specific tetrahedron chain stable in the monoclinic structure. The microwave dielectric permittivity was found to decrease linearly from 68 to about 26.6; meanwhile, the quality factor (Qf) value increased from 8000 GHz to around 23900 GHz as the x value increased from 0 to 0.60. The best microwave dielectric properties were obtained in a (Bi0.75Ce0.25)VO4 ceramic with a permittivity of ∼47.9, a Qf value of ∼18000 GHz, and a near-zero temperature coefficient of ∼+15 ppm/°C at a resonant frequency of around 7.6 GHz at room temperature. Infrared spectral analysis supported that the dielectric contribution for this system at microwave region could be attributed to the absorptions of structural phonon oscillations. This work presents a novel method to modify the temperature coefficient of BiVO4-type materials. This system of microwave dielectric ceramic might be an interesting candidate for microwave dielectric resonator and low-temperature cofired ceramic technology applications.

Development of Polymer-Ceramic-Metal Graded Acoustic Matching Layers via Cold Sintering.

A family of three phase, polymer-ceramic-metal (Poly-cer-met) electrically conducting composites was developed via cold sintering for acoustic matching application in medical ultrasound transducers. A range of acoustic impedance ( Z ) between MRayl with low attenuation (<3.5 dB/mm, measured at 10 MHz) was achieved in composites of zinc oxide, silver, and in thermoplastic polymers like Ultem polyetherimide (PEI) or polytetrafluoroethylene (PTFE) at sintering pressure less than 50 MPa and temperature of 150 °C. Densities exceeding 95% were achieved, with resistivities less than 1 Ω -cm. The acoustic velocity was homogeneous across the part (variations <5%). The acoustic velocities exceeded 2500 m/s for Z above 12 MRayl. The experimentally measured acoustic impedance of ZnO/Ag/PEI composites was observed to be in close agreement with the theoretical logarithmic model developed for different volume fractions of individual phases at the percolation limit for Ag. Thus, the acoustic properties of this family of matching layers (MLs) can be predicted to a good approximation before experimental realization. Additionally, a non-conducting low Z (5 MRayl MRayl) with acoustic velocities exceeding 2000 m/s was achieved using hydrozincite as the ceramic component. Scaling of the composites to 2'' diameter was demonstrated. A -6 dB bandwidth greater than 85% was measured for a three ML ultrasound transducer, fabricated using a single cold sintered layer ( Z = 19 MRayl) and two other commercial layers in the stack. Finally, a co-cold sintered graded prototype consisting of three tape-casted formulations corresponding to Z = 5 , 9, and 19 MRayl, while still retaining the correct distributions of the components was demonstrated.

Design and Sintering of All-Solid-State Composite Cathodes with Tunable Mixed Conduction Properties via the Cold Sintering Process.

Electrodes for solid-state batteries require the conduction of both ions and electrons for extraction of the energy from the active material. In this study, we apply cold sintering to a model composite cathode system to study how low-temperature densification enables a degree of control over the mixed conducting properties of such systems. The model system contains the NASICON-structured Na3V2(PO4)3 (NVP) active material, NASICON-structured solid electrolyte (Na3Zr2Si2PO12, NZSP), and electron-conducting carbon nanofiber (CNF). Pellets of varying weight fractions of components were cold-sintered to greater than 90% of the theoretical density at 350-375 °C, a 360 MPa uniaxial pressure, and with a 3 h dwell time using sodium hydroxide as the transient sintering aid. The bulk conductivity of the diphasic composites was measured with impedance spectroscopy; the total conductivities of the composites are increased from 3.8 × 10-8 S·cm-1 (pure NVP) to 5.81 × 10-6 S·cm-1 (60 wt % NZSP) and 1.31 × 10-5 S·cm-1 (5 wt % CNF). Complimentary direct current polarization experiments demonstrate a rational modulation in transference number (τ) of the composites; τ of pure NVP = 0.966, 60 wt % NZSP = 0.995, and 5 wt % CNF = 0.116. Finally, all three materials are combined into triphasic composites to serve as solid-state cathodes in a half-cell configuration with a liquid electrolyte. Electrochemical activity of the active material is maintained, and the capacity/energy density is comparable to prior work.

ReaxFF Molecular Dynamics Study on the Influence of Temperature on Adsorption, Desorption, and Decomposition at the Acetic Acid/Water/ZnO(101Ì 0) Interface Enabling Cold Sintering.

The reaction dynamics of a liquid-solid interface with the example of an acetic acid/water solution interacting with a ZnO(101̅0) surface was investigated using ReaxFF reactive force field-based molecular dynamics. The interactions were studied over a broad temperature range to assess the kinetics and reaction pathways. Two different acetic acid dissociation mechanisms are observed in the simulations: (1) deprotonation to surface cation, which produces a terminal hydroxyl and (2) deprotonation to a bridging hydroxyl, which results in water production. An increase in temperature promotes the dissociation of acetic acids and its adsorption to surface at first, but as the temperature increase continues, the surface coverage by acetates decreases due to evaporation from the surface or decomposition. The acetate decomposition starts with a nucleophilic attack of oxygen to methyl carbon and results in the production of carbon dioxide, which is consistent with experimental findings in the literature. The coverage of the surface by water molecules decreases as the system is heated up, which is also observed in other molecular dynamics studies. At elevated temperatures, acetate molecules are more stable than water molecules or bridging hydroxyls on the surface. These simulations validate the ReaxFF method for the water/organic mixture and metal oxide surface interactions and provide insights into structure and reactivity of aqueous solvents on metal oxide surfaces at elevated temperatures. Adsorption trends that are observed in this study are consistent with phenomenological Langmuir models. The reaction of acetic acid decomposition to smaller molecules such as CO2 and CH2O agrees with experimental observations. Understanding the details of these dynamic surface reactions are critical to understand important new cold sintering processes that utilize transient liquid and solid reactions, and the latter could be used to predict solvent selection for cold sintering.

Cold Sintered Ceramic Nanocomposites of 2D MXene and Zinc Oxide.

Nanocomposites containing 2D materials have attracted much attention due to their potential for enhancing electrical, magnetic, optical, mechanical, and thermal properties. However, it has been a challenge to integrate 2D materials into ceramic matrices due to interdiffusion and chemical reactions at high temperatures. A recently reported sintering technique, the cold sintering process (CSP), which densifies ceramics with the assistance of transient aqueous solutions, provides a means to circumvent the aforementioned problems. The efficacious co-sintering of Ti3 C2 Tx (MXene), a 2D transition carbide, with ZnO, an oxide matrix, is reported. Using CSP, the ZnO-Ti3 C2 Tx nanocomposites can be sintered to 92-98% of the theoretical density at 300 °C, while avoiding oxidation or interdiffusion and showing homogeneous distribution of the 2D materials along the ZnO grain boundaries. The electrical conductivity is improved by 1-2 orders of magnitude due to the addition of up to 5 wt% MXene. The hardness and elastic modulus show an increase of 40-50% with 0.5 wt% MXene, and over 150% with 5 wt% of MXene. The successful densification of ZnO-MXene nanocomposite demonstrates that the cold sintering of ceramics with 2D materials is a promising processing route for designing new nanocomposites with a diverse range of applications.

Li2CO3-Coated Ni Particles for the Inner Electrodes of Multilayer Ceramic Capacitors: Evaluation of Lifetime.

In previous work, it was demonstrated that using Li2CO3-coated Ni particles in the manufacturing of multilayer ceramic capacitor (MLCC) devices could improve both the permittivity and dissipation factors. However, adding Li+ ions to the system gave rise to the concern that ions could migrate under sustained electrical fields and thereby increase the degradation rates of the insulation resistance in MLCCs. In this paper, thermally stimulated depolarization current and highly accelerated lifetime testing were both utilized to evaluate the oxygen vacancy space-charge regions and migration in MLCCs. The results suggested that three parameters (the sintering schedule, Li2CO3 coatings, and oxygen flow during sintering) determine the overall resilience to the degradation. The Li+ ions did not migrate during degradation, as verified by time-of-flight secondary-ion mass spectrometry mapping; however, the Li ions enter the perovskite structure as an acceptor and, if ionically compensated for, could introduce more oxygen vacancies to the system and decrease the lifetime of the MLCCs. Nevertheless, it was demonstrated that the relative lifetimes of the newly designed MLCCs significantly improve relative to the conventional samples.

Redesigning Multilayer Ceramic Capacitors by Preservation of Electrode Conductivity and Localized Doping.

Both Li2CO3-coated nickel particles and fast firing technique were utilized in the manufacturing of MLCCs. They preserved the conductivity of Ni electrodes and provided the possibility of sintering the devices in oxidizing atmospheres. By using our method, the partial pressure of oxygen increased from 10-10 atm in conventional methods to 10-4 atm. The oxidizing atmosphere reduced the oxygen vacancy concentration as illustrated by the color change of the samples, and the results of EELS (electron energy loss spectroscopy). A systematic test with variable parameters, Li2CO3 coating, the sintering schedule, and the oxygen flow during sintering were executed, and the dissipation factor and the capacitance of the MLCCs were documented. Three type of MLCCs were studied: Conventional (fired with the conventional technique), Uncoated (fast fired with uncoated Ni particles), and Coated (fast fired with the coated Ni particles). The maximum oxygen activity during sintering (i.e., pO2 = 1.2 × 10-4 atm) was obtained for coated samples, and due to the minimum VO•• concentration, their dissipation factor decreased up to 60% relative to the Conventional ones. In addition, the impedance spectroscopy, together with the map of Li ion distribution, suggested that Li ions accumulated around the electrode-dielectric interface and amplified the activation energy at these interfaces. This eventually caused the coated MLCCs to show higher capacitance than their uncoated counterparts. As a conclusion, it is shown that the manufacturing process described in this paper can provide a better MLCC with higher capacitance, and lower dissipation factor and leakage current.

Protocol for Ultralow-Temperature Ceramic Sintering: An Integration of Nanotechnology and the Cold Sintering Process.

The sintering process is an essential step in taking particulate materials into dense ceramic materials. Although a number of sintering techniques have emerged over the past few years, the sintering process is still performed at high temperatures. Here we establish a protocol to achieve dense ceramic solids at extremely low temperatures (<200 °C) via integrating the particle nanotechnology into the recently developed cold sintering process (CSP). The sintering path has been appropriately tailored via effectively utilizing the large surface-to-volume ratio of nanoparticles. BaTiO3 ceramics have been used for the illustration, given its importance in extensive electronic device applications, as well as its scientific interest, being a model material for many of the ferroelectric materials. Together with detailed experimental studies, the trends are also analyzed with a fundamental thermodynamic consideration. Such an impactful technique could have widespread application prospects in a wide variety of materials and would also provide a clear roadmap to guide future studies on ultralow-temperature ceramic sintering, ceramic materials related integration, and sustainable manufacturing practices.

Hydrothermal-Assisted Cold Sintering Process: A New Guidance for Low-Temperature Ceramic Sintering.

Sintering is a thermal treatment process that is generally applied to achieve dense bulk solids from particulate materials below the melting temperature. Conventional sintering of polycrystalline ceramics is prevalently performed at quite high temperatures, normally up to 1000 to 1200 °C for most ceramic materials, typically 50% to 75% of the melting temperatures. Here we present a new sintering route to achieve dense ceramics at extraordinarily low temperatures. This method is basically modified from the cold sintering process (CSP) we developed very recently by specifically incorporating the hydrothermal precursor solutions into the particles. BaTiO3 nano polycrystalline ceramics are exemplified for demonstration due to their technological importance and normally high processing temperature under conventional sintering routes. The presented technique could also be extended to a much broader range of material systems than previously demonstrated via a hydrothermal synthesis using water or volatile solutions. Such a methodology is of significant importance, because it provides a chemical roadmap for cost-effective inorganic processing that can enable broad practical applications.

Investigation of Electric Field-Induced Structural Changes at Fe-Doped SrTiO3 Anode Interfaces by Second Harmonic Generation.

We report on the detection of electric field-induced second harmonic generation (EFISHG) from the anode interfaces of reduced and oxidized Fe-doped SrTiO3 (Fe:STO) single crystals. For the reduced crystal, we observe steady enhancements of the susceptibility components as the imposed dc-voltage increases. The enhancements are attributed to a field-stabilized electrostriction, leading to Fe:Ti-O bond stretching and bending in Fe:Ti-O6 octahedra. For the oxidized crystal, no obvious structural changes are observed below 16 kV/cm. Above 16 kV/cm, a sharp enhancement of the susceptibilities occurs due to local electrostrictive deformations in response to oxygen vacancy migrations away from the anode. Differences between the reduced and oxidized crystals are explained by their relative oxygen vacancy and free carrier concentrations which alter internal electric fields present at the Pt/Fe:STO interfaces. Our results show that the optical SHG technique is a powerful tool for detecting structural changes near perovskite-based oxide interfaces due to field-driven oxygen vacancy migration.