Near-Infrared Ratiometric Luminescent Thermometer Based on a New Lanthanide Silicate

A new lanthanide silicate system, Na₂ K[Ln₃ Si₆ O₁₈ ] (Ln=Lu, Yb/ Er, Lu/Eu, or Lu/Yb/Er), comprising microcrystals embedded in an amorphous siliceous matrix, obtained by sintering at 1373 K a Na₃ K[Ln₂ Si₆ O₁₇ ]⋅3 H₂ O nano-crystalline precursor, is reported. The crystal structure of these lanthanide silicates was solved from high-resolution synchrotron power X-ray diffraction data collected at 110 K, and further supported by ²⁹ Si MAS NMR and Eu³⁺ luminescence. The materials crystallize in the Pī triclinic centrosymmetric space group, exhibiting a dense framework consisting of hexameric [Si₆ O₁₈ ]^12- cyclosilicate units, and chains of two distinct {LnO₆ } octahedra. Na₂ K[(Lu_0.75 Yb_0.20 Er_0.05 )₃ Si₆ O₁₈ ] is the first example of a lanthanide silicate operative as a near-infrared ratiometric luminescent thermometer, with good sensitivity at cryogenic temperatures (<100 K). Upon excitation at 903 nm, the ratio between the ² F_7/2 →² F_5/2 (Yb³⁺ ) and ⁴ I_13/2 →⁴ I_15/2 (Er³⁺ ) emissions was used for sensing temperature in the 12-450 K range, reaching a maximum thermal sensitivity of 2.6 % K^-1 at 26.8 K.