Intrinsic defects and conduction characteristics of Sc2O3in thermionic cathode systems

Recent experimental observations indicate that bulk Sc${}_{2}$O${}_{3}$ (\ensuremath{\sim}200 nm thick), an insulator at room temperature and pressure, must act as a good electronic conductor during thermionic cathode operation, leading to the observed high emitted current densities and overall superior emission properties over conventional thermionic emitters which do not contain Sc${}_{2}$O${}_{3}$. Here, we employ ab initio methods using both semilocal and hybrid functionals to calculate the intrinsic defect energetics of Sc and O vacancies and interstitials and their effects on the electronic properties of Sc${}_{2}$O${}_{3}$ in an effort to explain the good conduction of Sc${}_{2}$O${}_{3}$ observed in experiment. The defect energetics were used in an equilibrium defect model to calculate the concentrations of defects and their compensating electron and hole concentrations at equilibrium. Overall, our results indicate that the conductivity of Sc${}_{2}$O${}_{3}$ solely due to the presence of intrinsic defects in the cathode operating environment is unlikely to be high enough to maintain the magnitude of emitted current densities obtained from experiment, and that the presence of impurities is necessary to raise the conductivity of Sc${}_{2}$O${}_{3}$ to a high enough value to explain the current densities observed in experiment. The necessary minimum impurity concentration to impart sufficient electronic conduction is very small (approximately 7.5 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}3}$ ppm) and is probably present in all experiments.