In‐situ Optical Investigation of Oxygen Diffusion Profiles in SrTi0₃

Abstract

The time evolution of oxygen stoichiometry profiles in Fe-doped SrTiO, single crystals has been detected and analyzed in-situ using an optical absorption technique. The profiles can be fitted according to Fick's second law by a constant chemical diffusion coefficient. The data obtained are in good agreement with earlier results from integral detection techniques and can be quantitatively understood by including the influence of internal coupling reactions to bare ambipolar transport via short range interaction.lntroduction ln this communication we report on an in-situ and contactless determination of oxygen diffusion profiles and on a precise determination of the chemical diffusion coefficient of oxygen (Do) in SrTi03. To our knowledge this is the first example of such a direct measurement of the space and time behaviour of diffusion kinetics of oxygen exchange. The method has been developed from earlier variants in which either the integral response had been measured in-situ at elevated temperatures or the profile had been frozen-in [l 1. Integral absorbance measurements have been reported in Ref.[I] (on SrTi03) and [2] (on NiO), and frozen-in profiles in reduced SrTi03 [3]. Closest to our approach is a report by Ben-Michael and Tannhauser [4] on the in-situ investigation of a colour front upon reduction of Zr02. There, however, profiles could not be evaluated.The knowledge of the chemical diffusion coefficients of oxygen in oxides is of great interest for both fundamental and technological reasons. This parameter determines or at least limits the overall rate of chemical reactions; it determines leakage rates in ion-conducting membranes, permeation rates in chemical filters, response times in bulk conductivity sensors, or drift effects in boundary conductivity sensors. Surprisingly, even the chemical diffusion of such intensively investigated materials as SrTi03 or stabilized Z r 0 2 is not fully understood yet. During our experimental and theoretical work it turned out that the coupling of internal defect chemical equilibria to the diffusion provides the clue for an adequate description [ 1, 51.Our decision to focus primarily on SrTi03 had a twofold origin and exhibits a basic and a technological aspect. On the one hand, the bulk equilibrium defect chemistry is wellunderstood, and even subtle details can be evaluated [5]. On the other hand, chemical diffusion in SrTi03 is crucial with respect to the performance of oxygen sensor and the durability of a perovskitic capacitor or actuator materialsThe basic oxygen incorporation reaction can be formulat-[6-81.ed as (1) 1 -02(g)+VG * 0 & + 2 h ' .2In Eq. (1) Vt;, 0 x 0 , and h' denote an effectively twofold positively charged oxygen vacancy, an effectively neutral regular oxygen ion, and an electron hole, respectively (Kroger-Vink notation). The mass action constant for Eq. (1) is taken as [5] The defect chemistry of nominally pure perovskite materials such as SrTi03 is greatly influenced by dopants, mostly transition metal ions. Therefore we have used deliberately ...

First-principles study on migration mechanism in SrTiO3

“…1. 9,11 It is also noted that this calculated migration energy is consistent with previously reported experimental and calculated values. Given that the shape of the supercells is cubic, the dependence on the migration direction was not considered.…”

“…Migrations to the first nearest-neighbor vacancy sites for the respective ions were considered. [7][8][9][10][11][12] On the other hand, the migration energy of Ti was calculated to be 8.92 eV, which is extremely high compared to that of Sr or oxygen. The calculated energies for the respective ions are plotted in Fig.…”

“…[7][8][9][10][11][12] The diffusion of SrTiO 3 influences grain growth and sintering behaviors; in addition, it has recently been the focus of attention because of its close relationship to the intermixing behaviors at heterointerfaces in SrTiO 3 -based superlattices. [7][8][9][10][11][12] The diffusion of SrTiO 3 influences grain growth and sintering behaviors; in addition, it has recently been the focus of attention because of its close relationship to the intermixing behaviors at heterointerfaces in SrTiO 3 -based superlattices.…”

High-Temperature 2D Optical Relaxation Visualizes Enhanced Oxygen Exchange Kinetics at Metal-Mixed Conducting Oxide Interfaces

“…As an alternative to measurements with current collectors, optical methods provide a contact-free approach. Continuous, in situ evaluation of a material’s optical absorption or transmission can be applied as a probe of defect dynamics to study many of the same properties: defect concentrations (stoichiometry), diffusivities, and surface reaction kinetics. , For example, in certain mixed conducting oxide compositions, it has been shown that increases in concentrations of oxide ions and holes in the oxides are linearly proportional to the increase in optical absorption at key wavelengths. − Thus, optical transmission relaxation (OTR) has emerged as a time-domain technique that can quantify, for example, oxygen surface exchange coefficients ( k chem ), which relate the oxygen flux to the driving force (electrochemical potential gradient) and serve as effective rate constants for the surface oxygen incorporation/evolution process: 1 2 normalO 2 ( g ) + normalv normalO • • ↔ normalO normalO × + 2 normalh • …”

In Situ Optical Absorption Studies of Point Defect Kinetics and Thermodynamics in Oxide Thin Films

“…In the in situ optical absorption relaxation experiments described by I. Denk et al 212 with Fe-doped SrTiO 3 , the experimental quantity measured was the relative change in optical absorbance at 440 and 590 nm corresponding to the change in and thus a change in . Equations ( 9) and ( 10) in section 4 express the change in absorbance and transmitted light intensity as a function of oxygen concentration, relevant to this experiment.…”