Polymorphism of NaYF_4: Yb~(3+), Er~(3+) up-conversion luminescence materials

摘要

The NaYF_4:Yb~(3+),Er~(3+) (x_(yb): 0.17, xEr: 0.03) materials were prepared by the co precipitation method. The crystal structure of an as-prepared material as well as of those annealed at 400 and 700°C were investigated at room temperature and in situ at higher temperatures by X-ray powder diffraction (XPD) and Rietveld analyses. The as-prepared material as well as that annealed at 700°C were found to crystallize with the cubic unit cell (Fm3m, No. 225, Z: 2) with a: 5.48 ?. The Rietveld results suggest that both structures are rich in rare earth ?, but the high temperature form has an even higher R~(3+) dominance over Na~+ (Na/R ratio: 0.64). The low-temperature cubic structure is similar to the high-temperature one, but the former shows a displacement of the cations. Heating at 400°C resulted in the hexagonal form (P6, No. 174, Z: 1.5 or P6_3/m, No. 176, Z: 1) with a: 5.978 and c: 3.501 A. For this structure, the Na/R ratio is closest to unity (0.92).