Temperature sensing by means of a near-infrared luminescent Ca8NaBi(PO4)6F2:Nd3+ fluorapatite phosphor

Abstract Microrods of calcium sodium bismuth fluorapatite doped with neodymium, denoted as Ca 8 NaBi 1-x Nd x (PO 4 ) 6 F 2 (with x ranging from 0 to 0.5), were synthetized using a modified Pechini sol–gel method. The crystalline structure of these phosphors was refined using the Rietveld method and exhibited hexagonal symmetry with space group P6 3 /m—C 2 6h , where the lattice parameters were a = 9.3855(5) Å and c = 6.8998(6) Å for x = 0.2. The particles morphology was analyzed through SEM, revealing an average length of approximately 1.5 µm. When excited at 808 nm, the Ca 8 NaBi(PO 4 ) 6 F 2 :0.2Nd 3+ microrods emitted strongly at 872, 957 and 1055 nm, falling within the near-infrared region. These emissions correspond to the 4 F 3/2 → 4 I 9/2 (P2), 4 F 5/2 → 4 I 11/2 (P3), 4 F 3/2 → 4 I 11/2 (P1) transitions of Nd 3+ ions, respectively, and are situated within the first and second biological windows. The luminescence lifetime of the 4 F 3/2 state of Nd 3+ was measured to be 294.4 µs for the sample with the lowest Nd 3+ concentration of x = 0.05. In addition, the luminescence intensity ratios P2/P1 and P3/P1 were found to be temperature dependent, potentially making it suitable for luminescent ratiometric thermal sensing. These findings suggest that the synthesized Ca 8 NaBi(PO 4 ) 6 F 2 :Nd 3+ microrod thermometers exhibit favorable characteristics in terms of relative sensitivity, temperature uncertainty, and repeatability within the temperature range of 303–403 K.