Sol-gel synthesis and characterization of fluoride-rich lanthanum-alumino-silicate gels doped with Ce3+ and Ti4+

A series of lanthanum- alumino- silicate gels doped with Ce3+ (0.5- 10.0 mol%) and Ti4+ were synthesized by a sol-gel process using trifluoroacetic acid (TFA) as a fluorine source. The structural (X- ray diffraction, scanning electron microscopy), thermal (differential scanning calorimetry), and optical (absorption, photoluminescence, photoluminescence-excitation) properties were investigated. A high fluorine content of up to 22.3 at.% was measured in the dried gels, significantly exceeding the 65 at.% fluorine content of earlier studies. The monolithic gels were transparent, amorphous, and stable up to 250- 300 degrees C. However, the gels lost their structural integrity at temperatures above 315 degrees C due to the thermal decomposition of TFA. The sol-gel route using TFA as a fluorine source is therefore not a viable route for the preparation of nanostructured glass ceramics containing a high volume fraction of crystalline LnF(3). All Ce3+ -doped gels showed luminescence in the blue spectral region. Gels containing Ti4+ had an additional strong oxygen-to-metal charge transfer transition that competed with the Ce3+ optical excitation and led to overall lower emission intensity. The measured luminescence intensity of all gel compositions decreased with increasing Ce3+ concentration as a result of increased reabsorption of Ce3+ emission by other Ce3+ ions in the gel as well as energy migration among Ce3+ ions to quenching sites.