The synthesis, crystal structure and multicolour up-conversion fluorescence of Yb3+/Ln3+ (Ln = Ho, Er, Tm) codoped orthorhombic lutetium oxyfluorides

Abstract

For the first time, undoped and Yb³⁺/Ln³⁺ (Ln = Ho, Er, Tm) codoped orthorhombic lutetium oxyfluorides have been fabricated via a facile stepwise route involving room temperature co-precipitation and a subsequent thermal decomposition procedure. The phase formation, crystal structure, particle morphologies and up-conversion (UC) luminescence were investigated by means of powder X-ray diffraction, TEM, IR, thermal and luminescence spectroscopy. It has been found that the intermediate Lu(CO₃)_1−x(OH)_yF_1+2x−y precursor plays an essential role in the formation of the final metastable Lu_nO_n−1F_n+2 Vernier phase. Multicolour up-conversion has been achieved in the Yb³⁺/Tm³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Er³⁺ codoped samples under near-infrared excitation (λ = 980 nm) and, notably, the Yb³⁺/Er³⁺ codoping yields an almost single-band red emission at about 660 nm. A short Ln³⁺–Ln³⁺ distance combined with the multiple doping sites in the low symmetry (C₁ and C_s) host lattice are considered responsible for the effective energy transfer and, therefore, the intense up-conversion emission. The present work, together with previous studies on the UC performance of Vernier phase yttrium oxyfluorides, illustrates that the Vernier phase oxyfluorides are promising up-conversion host materials for lanthanide ions in a variety of applications such as bioprobes and chromatic displays.