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Recent experimental observations indicate that bulk $Sc_2O_3$ (~200 nmthick), an insulator at room temperature and pressure, must act as a goodelectronic conductor during thermionic cathode operation, leading to theobserved high emitted current densities and overall superior emissionproperties over conventional thermionic emitters which do not contain$Sc_2O_3$. Here, we employ ab initio methods using both semilocal and hybridfunctionals to calculate the intrinsic defect energetics of Sc and O vacanciesand interstitials and their effects on the electronic properties of $Sc_2O_3$in an effort to explain the good conduction of $Sc_2O_3$ observed inexperiment. The defect energetics were used in an equilibrium defect model tocalculate the concentrations of defects and their compensating electron andhole concentrations at equilibrium. Overall, our results indicate that theconductivity of $Sc_2O_3$ solely due to the presence of intrinsic defects inthe cathode operating environment is unlikely to be high enough to maintain themagnitude of emitted current densities obtained from experiment, and thatpresence of impurities are necessary to raise the conductivity of $Sc_2O_3$ toa high enough value to explain the current densities observed in experiment.The necessary minimum impurity concentration to impart sufficient electronicconduction is very small (approximately $7.5x10^{-3}$ ppm) and is probablypresent in all experiments.

Intrinsic defects and conduction characteristics of $Sc_2O_3$ in thermionic cathode systems