Doping effect of sodium on γ-irradiated magnesium oxide

Abstract

The concentration of one-electron donor centres increases markedly when γ-irradiated MgO is treated with sodium amide (NaNH₂), followed by calcination at 500 °C for 2 h. The drastic increase in the concentration of one-electron donor centres is attributed to the electron-donating effect of Na produced by the decomposition of NaNH₂ at the anionic vacancy sites which are formed during irradiation. In contrast, the concentration of one-electron donor sites decreases if NaNH₂ is previously introduced into MgO before γ-irradiation. A similar result is obtained with MgO doped with NaNO₃. Thus in both cases when NaNH₂ was doped into the irradiated MgO and when MgO–NaNH₂ was irradiated, the concentration of one-electron donor centres changed considerably. This is explained by transfer of electrons trapped at anion vacancies to lattice oxygen atoms in MgO. The electrical conductivity was measured at temperatures between 50 and 300 °C. As the sodium content increases the electrical conductivity also increases. The thermal activation energy, described by the conductivity, differs on going from the low- to the high-temperature region, indicating that there exist two kinds of electron levels. The correlation between electroconductivity, one-electron donor and basicity properties is discussed.