Structural, electronic and optical properties of BaRE6(Ge2O7)2(Ge3O10) (RE = Tm, Yb, Lu) compounds and BaYb6(Ge2O7)2(Ge3O10):Tm3+ and BaLu6(Ge2O7)2(Ge3O10):Yb3+,Tm3+ phosphors: potential applications in temperature sensing

A new series of BaRE₆(Ge₂O₇)₂(Ge₃O₁₀) (RE = Tm, Yb, Lu) germanates and activated phases BaYb₆(Ge₂O₇)₂(Ge₃O₁₀):xTm³⁺ and BaLu₆(Ge₂O₇)₂(Ge₃O₁₀):12yYb³⁺,yTm³⁺ have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P2₁/m, Z = 2). The crystal lattice consists of zigzag chains of edge-sharing distorted REO₆ octahedra, bowed trigermanate [Ge₃O₁₀] units, [Ge₂O₇] groups, and eight-coordinated Ba atoms. The density functional theory calculations have confirmed a high thermodynamic stability of the synthesized solid solutions. According to the results of vibrational spectroscopy studies and diffuse reflectance measurements, the BaRE₆(Ge₂O₇)₂(Ge₃O₁₀) germanates are promising compounds for the creation of efficient lanthanide ion activated phosphors. Under 980 nm laser diode excitation, the BaYb₆(Ge₂O₇)₂(Ge₃O₁₀):xTm³⁺ and BaLu₆(Ge₂O₇)₂(Ge₃O₁₀):12yYb³⁺,yTm³⁺ samples exhibit upconversion luminescence corresponding to the characteristic ¹G₄ → ³H₆ (455–500 nm), ¹G₄ → ³F₄ (645–673 nm) and ³H₄ → ³H₆ (750–850 nm) transitions in Tm³⁺ ions. Heating of the BaLu₆(Ge₂O₇)₂(Ge₃O₁₀):12yYb³⁺,yTm³⁺ phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673–730 nm, caused by ³F_2,3 → ³H₆ transitions. It has been revealed that the fluorescence intensity ratio between this band and the band at 750–850 nm may be used for temperature sensing. The absolute and relative sensitivities in the studied temperature range reach 0.021% K⁻¹ and 1.94% K⁻¹, respectively. [ABSTRACT FROM AUTHOR]