Pushing the Study of Point Defects in Thin Film Ferrites to Low Temperatures Using In Situ Ellipsometry

Abstract

point defects, imperfections, and higher dimensional extended defects are known to severely impact the overall functional properties. [3,4] As a matter of example, oxygen vacancies were shown to enhance oxygen conductivity in oxide-ion electrolytes [5] or to boost oxygen incorporation and catalytic activity in mixed ionic electronic conductors (MIECs) [6,7] while weakening electronic and magnetic order in ferromagnetic oxides. [8] Besides, the presence of heterogeneous and homogenous interfaces in thin films was shown to drastically impact defect concentrations in such layers, which gives rise to deviations from bulk defect chemistry and, eventually, to new and unexpected properties. [9-11] Therefore, the knowledge and quantification of the chemical reactions that dominate the defect concentration in oxide thin films (i.e., defect chemistry) is essential for understanding the material behavior and for engineering their properties. This is especially relevant at intermediate-to-low temperatures (below 500 °C), where a high electrochemical activity and the nanometric dimensions of the thin films allow the point defect equilibrium with the environment, [12] hampering the use of the high temperature defect chemistry model from the bulk counterpart. A paradigmatic example of the large effect of point defects on the functional properties of transitional metal oxides can be found in the La 1−x Sr x FeO 3−δ (LSF) model family. LSF compounds crystalize in a perovskite ABO 3 structure and find application in many renewable energy technologies, such as solid oxide fuel cells (SOFC), [13] or electrochemical and photoelectrochemical water splitting. [14,15] In LSF, the substitution of trivalent La by divalent Sr gives rise to the generation of electronic holes and/or oxygen vacancies for electronic compensation, depending on the electrochemical equilibrium with the oxygen partial pressure of the environment. Interestingly, both point defects were found to be strongly correlated to many functional properties of LSF. For instance, the increase of holes concentration was found responsible for a large modification of the electronic structure, [16] affecting not just the electronic and magnetic transport properties [17] but also the oxygen evolution properties in aqueous media. [15] Meanwhile, oxygen vacancies were shown to take part into the rate-limiting step of oxygen incorporation at high temperature. [18] For these reasons, the development of a reliable and flexible in situ method for tracking the point defects of LSF thin films on any substrate and environment is fundamental for tailoring their properties. Unveiling point defects concentration in transition metal oxide thin films is essential to understand and eventually control their functional properties, employed in an increasing number of applications and devices. Despite this unquestionable interest, there is a lack of available experimental techniques able to estimate the defect chemistry and equilibrium constants in such oxides at intermediate-to-low temperatures. In ...

Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous Electrochemical Energy Storage Devices

“…In the N 2 -annealed sample, electronic charges are compensated by electron holes ([Sr La ′ ] = [Fe Fe • ], [V O •• ] ∼ 0), while in the O 2 -annealed sample electronic charges are compensated by oxygen vacancies ([Sr La ′ ] = 2­[V O •• ], [Fe Fe • ] ∼ 0). The change in optical conductivity observed for the O 2 -annealed and N 2 -annealed samples is consistent with the variation of holes in the LSF thin films, as observed in previous works. , Moreover, in our previous work, we directly observed a univocal association between the optical conductivity of LSF thin films and the electron hole concentration, which allowed us to quantify the defect chemistry of these layers under a variety of different electrochemical conditions . Thus, spectroscopic ellipsometry can be used to track the Fe 4+ /Fe 3+ transition through linking the optical conductivity with the concentration of electron holes.…”

“…KOH reduction clearly induces an intensity decrease of low-energy optical transitions (for A- and B-transitions with photon energies around 1 and 3 eV, respectively), while amplification of the one in the high-energy range (C-transition with photon energy around 4.2 eV). This behavior has been previously assigned to a strong modification of the LSF band structure, associated with the change in the concentration of Fe 4+ holes, which induces a redistribution of the optical weight and the creation of new intragap states upon hole doping. ,, To independently confirm this hypothesis, the optical conductivity spectra of LSF50 thin films fully oxidized and reduced in a conventional way (employing oxygen and nitrogen atmospheres at 575 °C) ,, were acquired and plotted in Figure b (named as “N 2 -annealed” and “O 2 -annealed”, respectively). In the N 2 -annealed sample, electronic charges are compensated by electron holes ([Sr La ′ ] = [Fe Fe • ], [V O •• ] ∼ 0), while in the O 2 -annealed sample electronic charges are compensated by oxygen vacancies ([Sr La ′ ] = 2­[V O •• ], [Fe Fe • ] ∼ 0).…”

“…In our previous work, the concentration of Fe Fe • in LSF has been directly related to the intensity of the low-energy transition “A” . In this way, the low-energy transition “A” can be used for calculating the hole concentration.…”

“…In this work, the concentration of Fe Fe • during the ion intercalation process was successfully measured by in situ ellipsometry on thin films. This in situ ellipsometry methodology has been recently proposed by the authors for successfully tracking defect chemistry and ion insertion in thin films under real electrochemical operation conditions. , Ellipsometry is a widely employed non-destructive spectroscopic technique, mainly used for the estimation of optical constants and thicknesses, which is based on the change of the polarization state of the light beam reflected on a thin film sample . In combination with ellipsometry, the mechanism of ion intercalation in LSF was investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS), positron annihilation lifetime spectroscopy (PALS), and ex situ ellipsometry.…”

Strain-Dependent Surface Defect Equilibria of Mixed Ionic-Electronic Conducting Perovskites

“…Therefore, to enable a more complete understanding of the surface defect chemistry and its strain dependency, one needs to reliably quantify the equilibrated surface defect concentrations in situ in a wide range of temperatures, gas pressures, and strain states. While significant advances have been made to enable in situ defect characterization, most techniques − are still bulk sensitive, making it difficult to isolate the surface defect chemistry. In this sense, near ambient pressure X-ray absorption spectroscopy (NAP-XAS) provides a powerful platform to probe the surface chemistry both in situ and in operando . , Since the sampling depths for the total/partial electron yield (TEY/PEY) NAP-XAS measurements are smaller than 5 nm, , the electronic structures revealed by PEY- and TEY-XAS should directly reflect the defect chemistry at the materials surfaces.…”