Electrical conductivity and defect structure of Mg-doped Cr sub(2)O sub(3)

The electrical conductivity of Cr sub(2)O sub(3) nominally doped with 2 mol% MgO has been studied by the four point a.c. technique as a function of the oxygen activity (O sub(2)+Ar, CO+CO sub(2) and H sub(2)+H sub(2)O) in the temperature range 400-1200 degree C. It is concluded that Cr sub(2)O sub(3) doped with MgO is an extrinsic conductor and that the dissolved Mg-dopant is compensated by the formation of electron holes at near atmospheric oxygen pressures and by oxygen vacancies (or possibly interstitial chromium ions) at highly reduced oxygen activities (in CO+CO sub(2) and H sub(2)+H sub(2)O gas mixtures). In H sub(2)+H sub(2)O mixtures Mg doped chromia also dissolves hydrogen as protons and significantly affects the defect structure and electrical conductivity. The defect structure of the oxide under various conditions is discussed.