Nonstoichiometry relaxations

Defect Structure, Nonstoichiometry, and Nonstoichiometry Relaxation of Complex Oxides... 

S0 I 70 Defect Structure, Nonstoichiometry, and Nonstoichiometry Relaxation of Complex Oxides the same as that for fixed a-noj. due to Le Chadier s principle ... 

With regards to the complex oxides, however, it is fair to say that the nonstoichiometry relaxation kinetics and chemical diffusion are, as yet, less well understood. In... 

These nonstoichiometry relaxations are satisfactorily described by Eq. (63), as depicted by the solid curves in Figure 10.12. The two kinetic parameters may subsequently be evaluated, as shown in Figure 10.13. 

Figure 10.12 Typical nonstoichiometry relaxation of undoped BaTiOsin its n-type branch of oxygen activity (a) and p-type branch of oxygen activity (b) during reduction and oxidation at 1000°C. The solid lines are best fitted to Eq. (63). Data from Ref [12],...

Finally, the true chemical diffusivities that are responsible for the diffusion step in the nonstoichiometry relaxation kinetics have emerged to be as shown in... 

Yoo HI (2010) Defect structure, nonstoichiometry, and nonstoichiometry relaxation of complex oxides. In Riedel R, Chen IW (eds) Ceramic science and technology, vol 2. Wiley-VCH, Weinheim... 

An isolated CS plane is referred to as a Wadsley defect and a random array of CS planes is considered to constitute planar (extended) defects which are entirely different from point defects. It is obvious that when CS planes occur at regular intervals, the composition of the crystal is stoichiometric, whereas a random array of CS planes results in nonstoichiometric compositions. While we have invoked anion vacancies which are later annihilated in our description of CS plane formation, we must point out that vacancies are not essential precursors for the formation of CS planes. Accommodating anion-deficient nonstoichiometry through CS mechanism is a special feature restricted to d° metal oxides such as W03, Nb205 and TiOz which exhibit soft phonon modes. Soft phonon modes in metal oxides arise from soft metal-orxygen potentials which permit large cation relaxation. The latter... 

Here, 8(f), 8(0), and 8(cx3) are the mean (at time f), initial (at f = 0) and final (as f —> CX3) value of oxygen nonstoichiometry, respectively. By monitoring the temporal variation of the nonstoichiometry 8(f) by either thermogravimetry or a 8-sensitive property (e.g., electrical conductivity), it is possible to determine the two kinetic parameters. With regards to binary systems, it is believed that the relaxation kinetics may be well understood. Chemical diffusion, in particular, has long been understood in the light of chemical diffusion theory [28], or in the light ofthe ambipolar diffusion theory [29]. 

When the oxygen partial pressure in the surrounding of the system oxide is abruptly changed, either in its n-type branch of oxygen activity (aei oc n oc or in the p-type branch (Oei oc p oc ct ) (see Figure 10.6a or b), its electrical conductivity, as a direct measure of the oxygen nonstoichiometry (6 w/2 or —p/2, respectively), will relax in typical fashion, as shown in Figures 10.12(a) and (b). 

The twofold relaxation is understood as follows. The nonmolecularity T as well as the oxygen nonstoichiometry 5 of the system is presumed, initially, to be spatially homogeneous. Then, as soon as a different oxygen activity is imposed upon such... 

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