X-ray absorption and diffraction studies of Pr³⁺, Tb³⁺ and Er³⁺-activated silica gels

Rare-earth (Pr³⁺, Tb³⁺, Er³⁺) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900-950°C and co-doping with aluminum ions was determined from the rare-earths L₃-edge EXAFS signals. The densification process induces a decrease of the coordination number and a compression and deformation of the first coordination shell, composed of oxygen atoms. The second coordination shell, composed of silicon and/or aluminum ions, also experiences some modification, which is attributed mainly to a shortening of the shell radius. No evidence of clustering of rare-earth ions upon densification was observed. X-ray diffraction data on Tb-doped gels confirm the EXAFS results.