Upconversion luminescence color modulation and temperature sensing of Na0.5Bi2.5Nb2-xTaxO9:Er3+/Yb3+ phosphors

A series of Na0.5Bi2.5Nb2-xTaxO9 (NBN2-xTx):Er3+/Yb3+ upconversion luminescence (UL) phosphors were prepared by a solid-state sintering method, and the lattice structure of the prepared samples was studied by X-ray diffraction. Under 980-nm laser excitation, the UL properties were investigated by a spectrophotometer from room temperature to 723 K, and the optimal concentration of co-doping Er3+/Yb3+ is 0.14 and 0.26 by replacing Bi3+ sites in NBN2-xTx host, respectively. With the increase of Ta5+ concentration, the color of UL gradually changes from green to yellow corresponding color coordinates from (0.301, 0.682) to (0.441, 0.549) under 980-nm laser excitation. This color change is attributed to the decrease of lattice unit volume leading to the increase of cross relaxation of Er3+ to Er3+ and energy back transfer from Er3+ to Yb3+. Using the luminescence intensity ratio technique, the maximum absolute sensitivities (S-A) are obtained as 0.00674 K-1 at 538 K and 0.00603 K-1 at 573 K, and relative sensitivities (S-R) are obtained as 0.01174 K-1 at 303 K and 0.01244 K-1 at 303 K based on H-2(11/2) -> I-4(15/2) and S-4(3/2) -> I-4(15/2) energy level transitions of Er3+ in NBN2:0.14Er(3+)/0.26Yb(3+) phosphors under 980- and 1550-nm laser excitations, respectively. The results suggest that the NBN2-xTx:Er3+/Yb3+ phosphors are promising candidates for temperature sensing with different UL colors.