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[(Lu0.75 Yb0.20 Er0.05 )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 F7/2 →2 F5/2 (Yb3+ ) and 4 I13/2 →4 I15/2 (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.