Changes in the Zr environment in zirconia–silica xerogels with composition and heat treatment as revealed by Zr K-edge XANES and EXAFS

Abstract

X-ray absorption spectroscopy at the Zr K-edge is an important technique for probing the environment of Zr. Here it is applied to zirconia–silica xerogels with composition 0.07⩽x⩽0.40, where x is the molar ratio Zr:(Zr+Si). Reference samples of crystalline ZrO₂, ZrSiO₄, BaZrO₃ and liquid Zr n-propoxide were also examined. New XANES (X-ray adsorption near edge structure) results are presented for zirconia–silica xerogels, and compared with previous EXAFS (extended X-ray absorption fine structure) results. For high Zr contents (x=0.4) there is a separate, amorphous ZrO₂ phase, which before heat treatment is similar to Zr hydroxide, and after heat treatment at 750°C is similar to an amorphous precursor of tetragonal ZrO₂. For low Zr contents (x=0.1) there is atomic mixing of Zr in the SiO₂ network, and the environment of Zr is more similar to that in Zr n-propoxide compared to other reference samples. New in situ XANES and EXAFS results are presented for x=0.1 xerogels heated at 250°C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.