Internal Reactions in Heterophase Assemblages

Heterophase assemblages of mixed ionic/electronic conductors of the type A/AX/AY/A under an electric load are the simplest inhomogeneous electrochemical systems that can serve to exemplify our problem. Let us assume that the transport of cations and electrons across the various boundaries occurs without interface polarization and that the transference of anions is negligible. For the other transference numbers we then have 

Therefore, if A(a = 0, the cation flux changes its density at the AX/AY interface. This means that this interface (by application of a sufficiently strong electric field) acts either as an A sink or as an A source depending on the direction of the A flux. In the first case, metallic A will be precipitated at the AX/AY interface. Since AtA = A/e the difference in electric current, A7e, will supply the necessary electrons for the (internal) reduction of the A cations. In the second case, the AX/AY interface operates as an A source and the lattice molecules AX or AY will be decomposed. Consequently, either X(Y) atoms or X2(Y2) molecules are formed and the corresponding reactions read 

In any case, crystal lattices are destroyed by the field-driven decomposition. If the original AX/AY interface remains coherent, stresses develop which will consume some driving force. In other words, the AX/AY interface is then polarized. A determination of the amount (=jA/A-dt) of decomposed AX(AY) at the interface should give a very sensitive method to measure extremely small differences in the elec- 

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