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):xTm(3+) and BaLu6(Ge2O7)(2)(Ge3O10):12yYb(3+),yTm(3+) have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P2(1)/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):xTm(3+) and BaLu6(Ge2O7)(2)(Ge3O10):12yYb(3+),yTm(3+) samples exhibit upconversion luminescence corresponding to the characteristic (1)G(4) -> H-3(6) (455-500 nm), (1)G(4) -> F-3(4) (645-673 nm) and H-3(4) -> H-3(6) (750-850 nm) transitions in Tm3+ ions. Heating of the BaLu6(Ge2O7)(2)(Ge3O10):12yYb(3+),yTm(3+) phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673-730 nm, caused by F-3(2,3) -> H-3(6) 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.