Color-tunable luminescence and temperature sensing properties of Bi³⁺/Sm³⁺ or Bi³⁺/Eu³⁺ codoped Ba₂Gd₂Ge₄O₁₃...

Novel temperature-sensitive luminescent materials, Ba₂Gd₂Ge₄O₁₃ doped with Bi³⁺, Bi³⁺/Sm³⁺ or Bi³⁺/Eu³⁺ ions, have been prepared using a conventional solid-state reaction technique. The XRPD study has verified that all the synthesized germanates crystallize in the monoclinic system (space group C2/c, Z = 4). The crystal lattice of the compounds consists of zigzag chains of edge-sharing Me₂O₇ (Me = Bi³⁺, Eu³⁺ or Sm³⁺) dimers, [Ge₄O₁₃] units, and ten-coordinated Ba atoms. Under UV excitation the powders exhibit the luminescence corresponding to the ³P₁ → ¹S₀ (310-550 nm) transition in Bi³⁺ and ⁴G_5/2 → ⁶H (550-730 nm) in Sm³⁺ or ⁵D₀ → ⁷F (570-720 nm) transitions in Eu³⁺ ions. Heating of Ba₂Gd_1.94Bi_0.01Sm_0.05Ge₄O₁₃ and Ba₂Gd_1.89Bi_0.01Eu_0.1Ge₄O₁₃ phosphors leads to an irregular decrease in the intensity of the main emission lines. It has been found that the fluorescence intensity ratio between a wide band in the 310-530 nm region and peaks at longer wavelengths may be successfully used as a temperature-dependent characteristic. Absolute/relative sensitivity values for Ba₂Gd_1.94Bi_0.01Sm_0.05Ge₄O₁₃ and Ba₂Gd_1.89Bi_0.01Eu_0.1Ge₄O₁₃ germanates reach 0.19% K^-1/0.80% K^-1 and 2.21% K^-1/0.58% K^-1, respectively. The above parameters indicate that Ba₂Gd₂Ge₄O₁₃:Bi³⁺/Sm³⁺ or Bi³⁺/Eu³⁺ samples can be used as potential luminescent materials for non-contact temperature measurement.