Enhanced up-conversion luminescence in Na5Y9F32: Ho3+/Yb3+/Eu3+single crystals by introducing of Gd3+ions for highly sensitive temperature sensing

A series of Na5Y9F32 single crystals multi-doped with fixed Eu3+/Ho3+/Yb3+ (0.3mol%/0.5mol%/0.3mol%) and various Gd3+ from 0 to 1.5mol% were synthesized by a Bridgman technique. The patterns of X-ray diffraction (XRD) affirmed that the obtained crystals had pure Na5Y9F32 crystal phase at the present doping concentrations. The Gd3+ doping concentration-dependent up-conversion (UC) luminescence spectra of the prepared samples were investigated upon excitation of 980 nm laser. Two up-conversion emission bands of Ho3+ were observed, including green emission at 544 nm of the transition from 5F4/5S2 to 5I8level, and red emission at 658 nm from 5F5 to 5I8. The up-conversion intensities at both 544 nm and 658 nm emissions increased as gradual increment of Gd3+ doping concentration from 0 to 1.5mol%. The effect of Gd3+ on the crystal field of the crystal were explored by the fluorescent spectra of Eu3+ upon excitation of 394 nm light and the optical intensity parameters of Ho3+ ions. The reduction of lattice symmetry by Gd3+ doping resulted into the augmentation of 544 nm and 658 nm up-conversion emissions. The temperature sensing characteristics of Na5Y9F32: Eu3+/Yb3+/Ho3+/Gd3+ from 298 K to 473 K were explored by fluorescence intensity ratio (FIR) of I544nm/I658nm, and the Gd3+ doping was significantly beneficial for enhancement of the sensitivity. The absolute sensitivity and relative sensitivity were enhanced from 2.08 %K-1 to 2.29 %K-1, and 0.23 %K-1 to 0.74 % K-1 by 1.5mol% Gd3+ doping, respectively. The high sensitivity and high thermal stability make the Na5Y9F32: Eu3+/Ho3+/Yb3+/Gd3+ UC crystal be an ideal material for temperature sensing.