Electrical Properties of Single Crystals, Bicrystals, and Polycrystals of MgO

The high oxygen pressure conductivity of high-purity single-crystal magnesium oxide at high temperatures varied as the 1/4 power of the oxygen partial pressure with a 3 eV activation energy; the low pressure conductivity varied as the -1/6 power of the pressure with an activation energy of 4 eV. The predominant defects proposed are (1) holes and singly ionized Mg vacancies at high pressure and (2) electrons and doubly ionized oxygen vacancies at low pressures. The effect of impurities was noted. Changes in the grain boundary conductance relative to the crystal conductance of MgO were observed as a function of temperature and pressure, but not as a function of grain boundary orientation, transport direction, or total impurity content. Increased high pressure ionic conductivity in polycrystalline material offered possible evidence for increased grain boundary diffusion.