Tracer oxygen ions

The electrical conductivity of sapphire in a particular crystallographic direction was found to be 1.25mSm 1 at 1773 K. An independent experiment on the same material at the same temperature determined the oxygen tracer diffusion coefficient to be 0.4nm2s 1, the diffusion occurring by a vacancy mechanism. Do these data favour oxygen ion movement as the dominant charge transport mechanism (Relative atomic masses, A1 = 27 and 0=16 density of sapphire, 3980 kgm-3.)... 

The tracer diffusion coefficient for oxygen ions in a particular cubic stabilized zirconia is measured and found to fit the relationship... 

The 18 O-tracer studies of Gordon and Taube (1962) on the oxidation of U(IV) on Pb02 have shown that both oxygen ions in the product U02 + are derived from the oxide lattice. This result indicates an inner-sphere mechanism and is compatible with a binuclear U(IV) surface complex ... 

A vacancy diffusion coefficient D is also commonly introduced in the literature as the transport of oxygen ions can also be viewed as the transport of oxygen vacancies in the opposite direction. The relation between this vacancy diffusion coefficient and the self-diffusion coefficient or the tracer diffusion coefficient of oxygen ions is... 

The multitude of transport coefficients collected can thus be divided into self-diffusion types (total or partial conductivities and mobilities obtained from equilibrium electrical measurements, ambipolar or self-diffusion data from steady state flux measurements through membranes), tracer-diffusivities, and chemical diffusivities from transient measurements. All but the last are fairly easily interrelated through definitions, the Nemst-Einstein relation, and the correlation factor. However, we need to look more closely at the chemical diffusion coefficient. We will do this next by a specific example, namely within the framework of oxygen ion and electron transport that we have restricted ourselves to at this stage. 

Here D is the tracer diffusion coefficient Dv is the diffusion coefficient of oxygen vacancy / is the correlation factor N is the total volimie concentration of oxygen ions in the substance. Eqn. (9) includes only free oxygen vacancies, i.e. not associated with the dopant cation. The diffusion coefficient is defined by ionic mobility mechanism and may be expressed in terms of ion hopping parameters ... 

Figure 39. Secondary-ion maps of 0— (oxygen tracer) and Mn 0— (positional reference) of the patterned thin-film electrode shown in Figure 38 following polarization at —0.34 V at 700 °C. (a) Tracer map at the top surface of LSM/YSZ, showing selective incorporation into LSM. (b) Tracer map near the LSMA"SZ interface (acquired after ablation of LSM off the surface with Cs+), showing deep penetration of tracer into LSM. (Reprinted with permission from ref 230. Copyright 2000 Elsevier.)...

Figure 40. Secondary-ion ratio map ( 0/ 0) of YSZ in the cell shown in Figure 39 following removal of LSM by etching in acid, (a) Map at the exposed YSZ surface, (b) Local analysis of the tracer across the region A—B shown in a at various depths, achieved by Cs+ ion ablation, (c) Qualitative models for oxygen incorporation used to rationalize the results in b. (Reprinted with permission from ref 231. Copyright 2002 Elsevier.)...

One last point. In the reaction of uranium(IV) where it is convenient to do a tracer experiment because there is only one metal ion product, we have actually determined the number of oxygens transferred to the uranyl ion product from the chlorite, and this number corresponds to 1.3 oxygen per chlorite transferred to the uranium. This is consistent with the results we reported some years ago (5) on the oxidation of uranium (IV) with Pb02 and Mn02, where indeed more than one oxygen is transferred. In conclusion, we feel that we have some direct evidence for two-electron transfer in these reactions and the formation of a chlorine(I) intermediate followed by the formation of chlorate. 

The studies on Cu2 aO mentioned above concluded that CujO is a metal-deficient p-type semiconductor with cation vacancies. It was not established, however, which kinds of defects (Vcu, Vcu) were dominant and what the effect of Q (interstitial oxygen) was on non-stoichiometry. To clarify these points, Peterson and Wiley measured the diffusion coefficient, D, of Cu in Cu2 O, by use of "Cu as a tracer over the temperature range 700-1153 °C and for oxygen partial pressures, greater than 10 atm. It has been widely accepted that lattice defects play an important role in the diffusion of atoms or ions. Accordingly it can be expected that the measurement of D gives important information on the lattice defects. 

This mechanism corresponds to an SN2 displacement of water from the methyloxonium ion by the acid. How could you distinguish between this mechanism and the addition-elimination mechanism using heavy oxygen (180) as a tracer ... 

Compounds containing 02 cation 340 are colorless with the exception of 02+PtF6, which is red due to the PtF6 ion. The compound 02+PF6 decomposes slowly at 80°C862 and rapidly at room temperature, giving oxygen, fluorine, and phosphorous pentafluoride. 18F tracer studies on 02+BF4 have led to the conclusion that the mechanism of the decomposition involves the equilibrium [Eq. (4.216)] followed by a bimolecular decomposition of 02F.863... 

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