M′-RTaO4 Synthesis: Activation of the Precursor Oxides by the Reaction Flux

Abstract Studies of the M′- R TaO 4 ( R = Gd, Y, Lu) phosphor synthesis reaction show that, contrary to previous views, the flux (e.g., Li 2 SO 4 , LiCl, Na 2 SO 4 ) serves as an important reactant. Through a combination of techniques (DTA, TGA, XRPD, and mass balance), we find that close to its melting point, the flux reacts exothermically with the blended tantalum and yttrium oxides to give intermediate compounds. These reactive intermediates then combine to make M′- R TaO 4 and to regenerate the flux. Thus, the flux serves a dual purpose; it first activates the oxide reaction mixture and then serves as a classical flux in the growth of R TaO 4 crystals. In light of this discovery, a more appropriate term for these flux compounds is catalyst or reactive flux because they facilitate the synthesis of R TaO 4 through chemical reaction.