Gadolinium conduction electron

S. Llibke, H.-D. Wiemhofer, Electronic Conductivity of Gadolinium Doped Ceria, Ber. Bunsenges. Phys. Chem., 102, 642 (1998). 

For a higher conductivity, cerium dioxide can be doped either with gadolinium Cei cGd (02 or with samarium Cex jSmj02 doped cerias are quite stable chemicdly. In solid oxide fuel cells they lack the effect of interactions between the electrolyte and the cathode materials that would lead to the formation of poorly conducting compounds. However, doped cerias have an important defect in that at low oxygen partial pressures (such as those existing close to the anode) they develop a marked electronic conduction. This is entirely inadmissible for the electrolyte, as it leads to internal self-discharge currents and even to a complete internal short circuit. The electronic conduction comes about when Ce" ions in the lattice are partly reduced to Ce " ions, which creates the possibility for electrons to hop between ions of different valency. 

A simplistic picture of the situation is to have a relationship between the efifective moments of the f-element materials with the probable ion configuration. In this situation, localized f electrons in the metal would have the same moment as localized f electrons in a compound. The moment would depend on the number of such localized electrons regardless of the particular f-element s chemical form. Thus, the number of localized f electrons in Gd metal is seven (4f with three electrons in a ds conduction band), as it is in Cm metal (Sfconfiguration) there are also seven localized f electrons in both gadolinium and curium sesquioxides. Further, terbium and berkelium dioxides have seven localized f electrons. All six materials should have the same moment based on seven, unpaired free-ion electrons. 

Ball and Stevens (74) report that gadolinium-doped ceria is a good candidate for use as an alternative electrolyte when compared to zirconia, due to its higher conductivity at lower temperatures. However, doped ceria has a number of disadvantages, such as electronic conductivity and reduced strength. Results indicate that an increase in strength can be produced in the ceria by addition of zirconia particles that is dependent on the particle size and heat treatment. 

Gadolinium- or samarium-doped cerium dioxide provides the highest ionic conductivity in cerium dioxide-based materials owing to similar ionic radii of Gd +Z Sm + and Ce +. The main issue of doped cerium dioxide is the onset of electronic... 

In fact, the contribution of electronic conductivity can be ignored because it is quite small. The major contribution to Cp arises from the lattice (vibrational) component Cy, which varies linearly as a function of molar volume V in a series of isomorphous compounds. For the com-poimds imder study, Cy can therefore be estimated using volume-weighted interpolation (Westrum et al., 1989) and data on isomorphous diamagnetic substances as reference values. For instance, the Cy heat capacities measured for trivalent lanthanum, gadolinium, and lutetium compoimds were used as reference values. Subsequently, the Cm and Csch contributions were determined by subtracting the sum of the Cy and Cd contributions from the experimental Cp values. 

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