UV Raman spectroscopic studies on the mechanism of FeAlPO ₄-5 synthesis

FeAlPO ₄-5 samples with various iron contents were investigated by UV resonance Raman spectroscopy and UV-Vis spectroscopy. The Raman spectra of FeAlPO ₄-5 show four feature bands related to the framework iron at 630, 1060, 1140, and 1210 cm ^-1 when excited by 266 nm laser. It is found that only part of ferric ions could enter into the framework, while the others exist in the extra-framework in the form of six-coordinated state. This kind of iron gives characteristic Raman band at 285 cm ^-1. Combined with UV resonance Raman spectroscopy, UV-Vis spectroscopy, and X-ray diffraction, the crystallization process of FeAlPO ₄-5 (Al/Fe = 760) was studied. It is found that, in the early stages of the crystallization, the iron species exist in the form of six-coordinated state while attached to the end of the 1-dimensional AlPO ₄ chain. The Fe-O bonds in the six-coordinated state hamper the crystallization of AlPO ₄. The crystallization of the FeAlPO ₄-5 began when the reaction between the 1-dimensional AlPO ₄ chains occurred. At the same time, the six-coordinated ferric ions converted to the tetrahedral iron.