Correlation between local lattice distortions and up-/down-conversion luminescence of (Y,Al)NbO4:Yb3+/Er3+

Abstract

The changes in the crystallographic parameters of Y 0.79- z Al z Yb 0.18 Er 0.03 NbO 4 powders prepared with varying Al content were determined by Rietveld refinement, and their effects on the up- (UC) and down-conversion (DC) spectra were studied. The findings confirmed that the Al 3+ , Yb 3+ , and Er 3+ ions were completely incorporated at the Y 3+ sites. The substitution of the small Al 3+ ions led to the modification of the interatomic distance. As a result, the highest polyhedral distortion index of the YO8 dodecahedron ( D YO ) could be achieved at z = 0.2. Under infrared radiation, UC spectra demonstrated that a two-photon energy transfer (ET) UC process from Yb 3+ to Er 3+ was responsible for the green and red emissions, whose intensities increased dramatically by 160 and 166% at z = 0.2, respectively. This enhancement was a consequence of a large crystal-field asymmetry surrounding the Yb 3+ and Er 3+ ions, which resulted from the highest D YO at z = 0.2. In addition to D , the Yb‒Er interatomic distance was responsible for the enhanced UC emission, affecting the ET probability from Yb 3+ to Er 3+ . Under 379 nm excitation, the DC green emission of Er 3+ also showed the maximal intensity at z = 0.2 owing to the largest D YO . However, a correlation between the crystallographic variations and the DC emissions activated by [NbO 4 ] 3- niobates could not be fully elucidated owing to the complexity of the emission mechanism.