Electrochemically driven solid state reactions in magnesium wüstite

Abstract

This contribution is concerned with driven (electro-) chemical reactions in ternary ionic crystals. Two well-known processes occur simultaneously if one applies oxygen electrodes (Pt, O₂) to an (A, B)O–crystal. Under load, firstly kinetic cation demixing takes place, if the mobilities of A ions and B ions are different. Secondly, in mixed conductors, when the electronic transference numbers change with position, local internal oxidation or reduction will take place. Thus we can expect that the mixed conductor (A, B)O is electrolysed with the above mentioned (Pt, O₂) electrodes. Internal oxidation and reduction will occur if the applied electric field is high enough. This contribution reports on the observation of driven internal reactions in (Mg, Fe)O, where under load the oxidation reaction becomes visible by the formation of of spinel (Mg, Fe)₃O₄ and the reduction by the decolouration of an initially homogeneous brownish solid solution. It could also be shown that the interplay of the driven demixing and the subsequent internal reactions leads at the anode side of the crystal to periodic processes in space and time. The latter phenomenon was observed in situ and at temperature in an optical heating stage microscope.