Mixed Ionic Electronic Conduction in Solids
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MIXED IONIC ELECTRONIC CONDUCTION IN SOLIDS I. RIESS Physics Department,
Technion,
Haifa 32000,
Israel
ABSTRACT Chemical diffusion in mixed ionic electronic conductors (MIECs) occurs via ambipolar motion. It finds many applications which are reviewed. The partial ionic and electronic conductivities are also of interest. Methods for determining these quantities are reviewed emphasizing problems with the Hebb-Wagner polarization method and suggesting an alternative method. The use of MIECs for probing simultaneously the ionic and electronic driving forces and for producing p-n junctions is discussed. I.
MIXED IONIC ELECTRONIC CONDUCTORS
Mixed ionic electronic conductors are solids that conduct ions as well as electrons and/or holes. For the ionic conductivity to prevail, the solid must contain ions in the first place. It is therefore typical that the mixed-ionicelectronic-conductors (MIECs) are ionically bonded compounds. However mixed conduction can be found also in other materials for example in Li doped Si. The motion of ions occurs under two conditions: a) that there is a vacant site (a vacancy or an interstical site) into which the ion can move, b) that the motion from one site to Qnother is not inhibited i.e. that the potential barrier separating the two adjuscent possible sites is not too high (for a given temperature). It is customary to limit the use of the name MIEC to compounds in which the difference in the ionic and electronic conductivities is less than two orders of magnitude and for which both conductivities are not too small ai,agl>10-5 S/cm , though clearly this is not a strict definition. The ionic and electronic charge carriers can be introduced in three ways: a) Intrinsically by thermal excitation, b) by deviation from stoichiometry and c) by doping. This is summarized in Table 1 which includes also examples [1]. The driving forces for the charged species in a MIEC having a uniform temperature are the gradients of the elctrochemical potential, A, of the corresponding species. 2.
APPLICATIONS OF MIEC
a.
Chemical Reactions in
MIEC
When oxygen diffuses into YBa 2 Cu 3 0 6 +x to increase the oxygen concentration the stoichiometry of the compound changes. This is a chemical reaction that proceeds with a given rate characterized by the chemical diffusion coefficient. Local neutrality is maintained throughout the oxide during the diffusion process. This however does not necessarily mean that Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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TABLE I
Origin of electronic and ionic conductivities Cause of Conductivity
Case Conductivity
Intrinsic
Deviation
Doping
Example
from Stoilchinmetry
1 2
3
x
AgS,
Electronic
X
CeO5 ..
Ionic
x
Electronic
x
Electronic
Ionic
x
YBasCuO;.,
Ionic
4
Electronic
x
CeOs+UO,
Electronic
x
Li doped Si
Ionic
x
Ionic 5
7
x
Electronic
x
Ionic
x
Electronic
x
Stoicliometric CuoS
II
Ionic 8
Electronic
9
x
M?)
brnO1
0
*
x .
Electronic
Ionic
in Pd
x
Ionic 9
Doped CeO, x
a
Chemical stoichiomet
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