Self-Activated Vanadate Compounds Toward Realization of Rare-Earth-Free Full-Color Phosphors

Rare-earth-free phosphors based on vanadate compounds were investigated, where the vanadates included chloride vanadates ((M2VO4Cl)-V-II), pyrovanadates ((M2V2O7)-V-II), orthovanadates (M-3(II)(VO4)(2)) with divalent cations M-II of Mg, Sr, Ba, and Zn, and oxofluorovanadates (A(I)VOF(4)) with an alkali metal A(I). A chloride pyrolysis method and a liquid phase precipitation method were proposed for preparing the chloride vanadates and pyro- and orthovanadates, respectively. These vanadate compounds showed self-activated photoluminescence (PL) based on the VO4 clusters against the ultraviolet (UV) light irradiation. The colors of PL covered almost the whole visible-light region from blue to yellow as Sr2VO4Cl (deep blue), Ca2VO4Cl (sky blue), Ba2V2O7 (green), Sr2V2O7 (yellowish green), Zn-3(VO4)(2) (yellow), and Mg-3(VO4)(2) (yellow). A correlation was suggested from these compounds between the luminescent colors and the structural feature as the longer V-O distances in the VO4 tetrahedra in the crystal structures led to the longer wavelength in PL. This seemed to be also applicable for the oxofluorovanadates A(I)VOF(4) (A(I)=K and Cs) which contain the VOF4 polyhedra with one O2- ion and four F- ions as the ligands, as they exhibited the reddish PL.