Doubling the relative sensitivity of YNbO4:Sm3+ luminescence thermometer by observing 4G7/2 emitting level

Abstract

The value of the energy gap between the thermally coupled emission levels of trivalent lanthanides limits the relative sensitivities of Boltzmann-type luminescent thermometers. The values of the relative sensitivities further decrease as the temperature increases, making their use challenging at high temperatures. Here, for the first time, we used the higher-energy emitting level of Sm³⁺ (⁴G_7/2) to improve the relative sensitivity at high temperatures. We prepared a YNbO₄:Sm³⁺ (6 mol% doping) luminescence thermometry probe using a vibrational ball mill, which homogenized the precursors, and thermally treated them for solid-state reactions. X-ray diffraction measurements proved that the phosphor crystallized in a monoclinic fergusonite-beta-(Y) structure, C2/c(15) space group, with a calculated average crystallite size of 83 nm. Scanning electron microscopy revealed the polycrystalline powder, with particles of about a few tens of micrometers. Photoluminescence excitation and emission spectra were recorded at 186 K as well as at room temperature. The excited state lifetime of the ⁴G_5/2 level measured at 300 K is 0.42 ms. The emission spectra were recorded in the 300–650 K temperature range and analyzed using the luminescence intensity ratio method. The results demonstrated a twofold increase in relative sensitivity within the 500–650 K temperature range when compared to the first excited level (⁴F_3/2). The highest relative sensitivity at 300 K is calculated from the ⁴F_3/2 level to be 1.81 %K⁻¹, while in the high temperature region it reached 1.31 %K⁻¹ at 500 K (obtained from the ⁴G_7/2 level).