Nonstoichiometry defect diffusivities

M.H.R. Lankhorst and H.J.M. Bouwmeester, Determination of oxygen nonstoichiometry and diffusivity in mixed conducting oxides by oxygen coulometric titration. Part 11 Oxygen nonstoichiometry and defect model for Lao.8Sro.2Co03-8. J. Electrochem. Soc. submitted... 

In order to do so we must first evaluate the chemical diffusion coefficients of the pair of majority defects (e.g., V and h ) in the semiconducting oxide A O. The coupling of the defect fluxes (jh--2jV = 0) to maintain electroneutrality results in a chemical diffusion coefficient Dv. This controls the change in nonstoichiometry, <5( ,/)> through defect transport and reads... 

Crystal Self-Diffusion in Nonstoichiometric Materials. Nonstoichiometry of semiconductor oxides can be induced by the material s environment. For example, materials such as FeO (illustrated in Fig. 8.14), NiO, and CoO can be made metal-deficient (or O-rich) in oxidizing environments and Ti02 and Zr02 can be made O-deficient under reducing conditions. These induced stoichiometric variations cause large changes in point-defect concentrations and therefore affect diffusivities and electrical conductivities. 

The maximum deviations from stoichiometry in CdTe have been found to be about 1017 excess Te or Cd atoms per cc., and in PbTe about 4 X 1018 excess Pb and 1019 excess Te atoms per cc. Because these deviations from a 1 to 1 stoichiometry are so small (5 X 10-4 or less), they cannot be studied by chemical analysis, x-ray diffraction, or density measurements. Rather the extent of nonstoichiometry and the nature of the defects must be inferred from such quantities as obtained from electrical, optical, magnetic, and self-diffusion studies. 

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

Oxygen nonstoichiometry of the perovskites Lai rxB03 (B = Cr, Mn, Co, Fe) and its relationship with electrical properties and oxygen diffusion has been studied extensively [159-161]. T) ical nonstoichiometry data for Lai. r FeOs. and for some other perovskites as obtained from gravimetric analysis and cou-lometric titration are given in Fig. 10.11. At small oxygen deficiency, acceptor dopants are the majority defects. The charge neutrality condition then becomes. 

The present review will cover the phase equilibria of these ceramic nuclear fuels at high temperatures, and a summary will be given on their nonstoichiometric region, defect structure and thermodynamic data. In addition, diffusion and vaporization processes will be taken as representative phenomena eharacteristic of these materials at high temperatures, and these phenomena also will be reviewed in their relation to nonstoichiometry. 

Figure 10.7 shows how the nonstoichiometry alters the trend of defect-sensitive properties, such as the electronic electrical conductivity, thermoelectric power, and oxygen chemical diffusivity [10]. Thus, the control of oxygen nonstoichiometry during processing is very often crucial to ensure the required properties of an oxide, or functions of the device thereof. 

Point-defect ordering (e.g., vacancy-dopant pairs) leads to interesting complications. Preparation conditions themselves (e.g., oxygen partial pressure and temperature in oxides) thermodynamically define and control this defect content and structure. It is important to realize that point defects are thermodynamically allowed and defined they are not anomalous in the least. Therefore, undoped, high-purity compounds may exhibit sizable nonstoichiometry due to intrinsic point defects. Doping (intentional addition of an impurity) allows one to precisely control the point-defect content and nonstoichiometry and, thereby, the properties. Transport properties are influenced by the point defects. Electrical conduction (hole or electron transport) and solid state diffusion of atoms generally vary with the quantity and type of point defects. 

Deviations from stoichiometry and chemical diffusion in metal-deficit samples were studied at 973 to 1273K, for S activities of 0.1 to lOOOOPa, by using a microthermogravimetric technique. It was found that the non-stoichiometry was a function of temperature and the equilibrium S pressure thus indicating that the cation vacancies did not interact, were fully ionized and were randomly distributed throughout the crystal lattice. Re-equilibration-rate measurements of nonstoichiometry showed that the vacancy diffusivity (a direct measure of defect mobility) did not depend upon their concentration and could be described by ... 

Because of the charge equilibrium within the oxide, the electronic partial conductivity (electrons or holes) Oei and the ionic partial conductivity Oi are linked to the oxygen nonstoichiometry and, consequently, are two key parameters characterizing a MEEC. The methods developed for deconvoluting the ionic and electronic contributions to the total conductivity of a MIEC allow either the measurement of the partial conductivities, the diffusion coefficient of mobile defects or the transport numbers, either tei or hon- Only the methods involving a solid electrolyte cell are briefly described. Reviews papers can be referred [Heyne, 1982 Rickert, 1982 Riess, 1997 Weppner Huggins, 1978]. 

---