Electrical conductivity of magnesium oxide single crystal below 1200 K

Using 2-, 3- and 4-electrodes configurations the direct current conductivity of MgO single crystals of nominally highest purity (with respect to cation impurities), grown by arc-fusion, was studied in argon or oxygen between 500–1200 K with special reference to both hysteresis effects during heating and cooling cycles and conductivity phenomena which occur underneath the surface in a thin subsurface zone. The samples contained 250–2500 at.-ppm carbon and typically 800 at.-ppm hydrogen.Below 1000 K the low temperature (LT) conductivity mechanism is characterized by an activation energy of 1.1 ± 0.2eV, distinctly lower than that of the high temperature region (HT) approximately 2.4 eV. By annealing at 300 K the LT mechanism progressively builds up and causes a very pronounced conductivity increase between 700–900 K, unaffected or even enhanced by O₂. Above 900 K, O₂ decreases the conductivity.The LT mechanism is proposed to be due to defect electrons on anion sites corresponding to O^? in the O^2? structure which are a consequence of the presence of carbon and hydrogen dissolved in the MgO and formally derived from the dissolution of traces of CO₂ and H₂O J. Chem. Phys. Solids43, 129 (1982)].The dissolved carbon is known to segregate into the elastically relaxed subsurface zone (J. Chem. Phys. Solids43, 59 1982). The conductivity data suggest that between 700–900 K, defect species of C + 2O^?, the dipolar CO^?₂, which become strongly enriched in the subsurface zone upon annealing at 300 K, dissociate according to the equation CO₂^2?→CO^? + O^?, thus generating the defect electrons responsible for the LT mechanism.