Synthesis and Properties of Upconversion Phosphors Based on Double Molybdates of Rare-Earth Elements

A series of upconversion phosphors M+(Yb0.99Tm0.005Ho0.005)(MoO4)2 (M+ = Li, Na, K, Rb, Cs) was synthesized by solid-state method. PXRD analysis revealed a clear correlation between the alkali cation radius and the structural type: Li and Na compounds adopt tetragonal scheelite-type structures, while K and Cs crystallize in orthorhombic modifications. For Rb, the predominant phase is Rb-rich Rb5Yb(MoO4)4. DSC revealed distinct thermal behavior: Li and Na molybdates melt congruently, while K, Rb, and Cs compounds show polymorphism preceding melting. Under 980 nm excitation, all samples exhibit characteristic Tm3+ and Ho3+ upconversion emission. The relative emission intensities vary with the structural type, resulting in different CIE color coordinates. This systematic study establishes the structure–property relationship in double molybdate phosphors.