Portal de Periódicos da CAPES

Electrical conductivity of magnesium oxide single crystal below 1200 K

1983; Elsevier BV; Volume: 44; Issue: 3 Linguagem: Inglês

10.1016/0022-3697(83)90052-5

1879-2553

H. Kathrein, Friedemann Freund,

Electronic and Structural Properties of Oxides

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 O2− 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. Solids 43, 129 (1982)]. The dissolved carbon is known to segregate into the elastically relaxed subsurface zone (J. Chem. Phys. Solids 43, 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 CO22−→CO− + O−, thus generating the defect electrons responsible for the LT mechanism.