Structural, electronic and optical properties of BaRE 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ) (RE = Tm, Yb, Lu) compounds and BaYb 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ):Tm 3+ and BaLu 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ):Yb 3+ ,Tm 3+ phosphors: potential applications in temperature sensing.

A new series of BaRE 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ) (RE = Tm, Yb, Lu) germanates and activated phases BaYb 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ): x Tm 3+ and BaLu 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ):12 y Yb 3+ , y Tm 3+ have been prepared using a solid-state reaction. An XRPD study has revealed that the compounds crystallize in the monoclinic system (space group P 2 1 / m , Z = 2). The crystal lattice consists of zigzag chains of edge-sharing distorted REO 6 octahedra, bowed trigermanate [Ge 3 O 10 ] units, [Ge 2 O 7 ] 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 BaRE 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ) germanates are promising compounds for the creation of efficient lanthanide ion activated phosphors. Under 980 nm laser diode excitation, the BaYb 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ): x Tm 3+ and BaLu 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ):12 y Yb 3+ , y Tm 3+ samples exhibit upconversion luminescence corresponding to the characteristic 1 G 4 → 3 H 6 (455-500 nm), 1 G 4 → 3 F 4 (645-673 nm) and 3 H 4 → 3 H 6 (750-850 nm) transitions in Tm 3+ ions. Heating of the BaLu 6 (Ge 2 O 7 ) 2 (Ge 3 O 10 ):12 y Yb 3+ , y Tm 3+ phosphor with the optimal composition up to 498 K leads to the enhancement of a broad band at 673-730 nm, caused by 3 F 2,3 → 3 H 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.