The conventional method of preparing YAG:Tb requires repeated firing to form single-phase cubic material. When a SrF2, BaF2, YF3, or AlF3, or AlF3 is used as a reaction reagent, however, only a single firing produces single-phase cubic material. The solid-liquid phase reaction occurs during firing between the raw oxide materials as well as other by-products. X-ray diffraction patterns identify these by-products as YAM, YAP, YF3, YOF, and BaAl2O4 (SrAl2O4). The particle growth is facilitated by a process in which small YAG particles first gather in the contact area between large YAG particles where fluoride melts and forms a liquid film, then dissolve and finally precipitate when saturated to reduce the surface energy.