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

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 less than or equal to x less than or equal to 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 degrees 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 degrees C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.