Upconversion Yb³⁺/Er³⁺:La₂Ti₂O₇ phosphors for solid-state lighting and optical thermometry

The solid-state reaction was employed as a suitable method to produce orthorhombic Yb³⁺/Er³⁺ co-doped La₂Ti₂O₇ phosphors. The pump power and temperature dependencies of spectra of the Yb³⁺/Er³⁺ co-doped La₂Ti₂O₇ were studied for color tunability and optical thermometry. The pump power significantly affected the color coordinates of the phosphors. The upconversion green light transformed to the broad white light at higher pump power. A two-photon transition was proposed to explain the upconversion energy transfer process. The fluorescence intensity ratio technique was used to calculate the optical temperature sensor sensitivities. The ΔE energy difference obtained from the intensity ratios of the thermally coupled ²H_11/2 and ⁴I_15/2 energy levels of Er³⁺ are 763.64 ± 76.62 cm^-1, 777.77 ± 44.62 cm^-1, 640.71 ± 137.39 cm^-1 for La_1.95Yb_0.04Er_0.01Ti₂O₇, La_1.94Yb_0.04Er_0.02Ti₂O₇, and La_1.93Yb_0.04Er_0.03Ti₂O₇, respectively. Absolute temperature sensitivities of the phosphors decreased with increasing Er³⁺ concentration. The maximum absolute sensitivity was determined to be 0.51x10^-2 K^-1 at 553 K for La_1.95Yb_0.04Er_0.01Ti₂O₇, 0.64x10^-2 K^-1 at 571 K for La_1.94Yb_0.04Er_0.02Ti₂O₇, and 0.38x10^-2 K^-1 at 457 K for La_1.93Yb_0.04Er_0.03Ti₂O₇.