The variables which influence the surface composition of vaporizing compounds have been studied using the behavior of LaB6 as an example. The thermionic work function of LaB6 has been determined as a function of surface stoichiometry. A nearly constant value of 2.93 eV for the effective work function was observed from the LaB6+LaB4 boundary up to approximately LaB6.06. A further increase in boron content caused a steep increase reaching 3.40 eV at the LaB6+LaB9 boundary at 1500 K. The interior composition which produces a surface composition having a high emission flux and the lowest attrition rate is in the high boron region near LaB6.5, not at the stoichiometric composition.
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