“…Since the diffusion of sodium metal atoms in aluminum oxide is very unlikely, sodium must be in its oxidized state prior to penetrating the arc tube and forming beta alumina at spinel particles. In the case of calcium, the best thermodynamic data available for beta alumina (13) and calcium hexaaluminate (14) indicate that, in the presence of aluminum oxide, calcium hexaaluminate is a more stable compound. Therefore, in a HPS lamp, aluminum oxide is more likely to be reduced by sodium vapor than calcium hexaaluminate or, for that matter, magnesium aluminate spinel.…”