Crystal Structure, Luminescent Properties, and Prospects for Using Ba3La2–xTbx(Ge3O9)2 and Ba3La1.3Tb0.7–xEux(Ge3O9)2 in Optical Thermometry

New thermosensitive luminescent materials based on Ba₃La₂(Ge₃O₉)₂ doped with Tb³⁺ or Tb³⁺/Eu³⁺ have been studied. Ba₃La_2–xTb_x(Ge₃O₉)₂ and Ba₃La_1.3Tb_0.7–xEu_x(Ge₃O9)₂ solid solutions were synthesized by the solid-phase method. All samples crystallized in the monoclinic system (space group C2/c, Z = 4) according to the results of x-ray diffraction analysis. The luminescent properties of Ba₃La_2−xTb_x(Ge₃O₉)₂ germanates were studied with excitation by radiation with λ_ex = 377 nm. The spectra consisted of several bands in the range 470–640 nm due to transitions from excited ⁵D₄ to ⁷F_J (J = 3, 4, 5, 6) levels in Tb³⁺ ions. The maximum luminescence intensity was observed for the sample with x = 0.7. Co-doping of phases with Eu³⁺ ions led to the appearance of additional lines in the luminescence spectra in the orange-red region that were associated with the transitions ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) in europium ions. The color characteristics of a Ba₃La_1.3Tb_0.1Eu_0.6(Ge₃O₉)₂ sample were studied. The temperature dependences of the intensity ratio of the luminescent bands were investigated upon heating from 298 to 473 K. It was shown that heating the phosphor led to an uneven decrease in the intensity of the main emission lines. The maximum values of sensitivities of S_a (1.05%/K) and S_r (1.00%/K) were achieved at T = 298 K.