An embodiment of the invention includes a particle. The particle includes a first yttria core; and a fluoride salt coating on the first yttria core. The coating is sufficiently continuous to prevent a large number of sites where a second yttria core may come into contact with the first yttria core. Optionally, the particle has been heated in an oxidizing atmosphere to a temperature in the range of about 400° C. to about 750° C. Optionally, the particle is substantially free of at least one of carbon-containing species and water. Optionally, the fluoride salt is lithium fluoride. Optionally, the fluoride salt is aluminum fluoride.