Crystalline Size Effect on the Energy Transfer from Mo−O Groups to Eu³⁺ Ions in R₂MoO₆:Eu (R = La, Gd, and Y) Crystals

Three series of R₂MoO₆:Eu (R = La, Gd, and Y) phosphor powders were synthesized by using the sol−gel method, and then the powder was annealed at a gradually decreased temperature from 1200 to 700 °C. The structures and spectra of the as-prepared phosphors were characterized by powder X-ray diffraction and photoluminescence measurements, respectively. The profile of the Y₂MoO₆:Eu particles was observed by means of field emission scanning electron microscopy. The excitation of the Mo−O groups yields the emission of Eu³⁺ ions, indicating that an energy transfer process takes place from the Mo−O groups to Eu³⁺ ions. With a decrease in annealing temperature, the intensities of the excitation and emission peaks decrease; at the same time, the position of the Mo−O excitation band shifts to the short wavelength direction. The variation of the Mo−O excitation band is discussed from three aspects: crystalline size, coordination environment of Mo, and structural symmetry of the host crystal.