We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O-) in a vacuum from fluorine-substituted derivatives of 12CaO center dot 7Al(2)O(3) (C12A7) with a composition of (12-x) CaO center dot 7Al(2)O(3)center dot xCaF(2) (0 <= x <= 0.8). Unsubstituted C12A7 easily decomposed into 5CaO center dot 3Al(2)O(3) (C5A3) and 3CaO center dot Al2O3 (C3A) above 830 degrees C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F- ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 degrees C. The emitted anionic species from the x = 0.4 sample were dominated by O- ions (similar to 92%) together with a small amount of O-2(-) ions (similar to 4%) and F- ions (similar to 4%). The absence of an O-2 gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O- ions and electrons with a ratio of < 1/1. The O-2 gas supply markedly enhanced the O- ion emission, and suppressed the electron emission. A sustainable and high-purity O- ion emission with a current density of 11 nA cm(-2) was achieved at 830 degrees C with the supply of 40 Pa O-2 gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F- ion-substituted C12A7 is a promising material for higher intensity O- ion emission at higher temperatures. (c) 2009 Elsevier B.V. All rights reserved.