Ion exchange and fixation of rare-earth cation into expandable tetrasilicic fluorine mica

Rare-earth cation (Nd³⁺) are incorporated into the interlayer spaces between the silicate layers of synthetic fluorine mica, Na_0.665Mg_2.68(Si_3.98Al_0.02)O_10.02F_1.98, by conventional ion exchange reaction. Subsequent migration of the interlayer cations upon calcination into the vacant octahedra of 2:1 layers is followed by powder X-ray diffraction, diffuse-reflectance UV spectroscopy, and X-ray absorption spectroscopy as a function of calcination temperature. It is found from the spectroscopic analyses that the interlayer cations start to migrate into the octahedral vacant sites from 400°C through the hexagonal siloxane ring of the tetrahedral silicate layers. According to the Nd L_III-edge XANES spectra, the normalized absorption intensity gradually decreases while the FWHM increases with temperature, suggesting that the bonding character of rare-earth cations and silicate lattices evolves from ionic to covalent as the calcination temperature increases.