X-Ray Absorption Spectroscopic Studies on Layered FeWO ₄Cl and Its Lithium Intercalate

Lithium intercalation into FeWO₄Cl leads to the structural phase transformation from tetragonal to monoclinic. According to Fe K-edge X-ray absorption near edge structure (XANES) spectroscopic analysis, only the Fe³⁺ ion in the FeWO₄Cl lattice is reduced to Fe²⁺ with an increment of site symmetry from C_4v to O_h upon lithium intercalation. On the other hand, the white line intensity at the W L-edge XANES becomes slightly enhanced after intercalation, indicating an increase of the (W⁶⁺–O) bond covalency due to a decrease of the competing (Fe²⁺–O) bond strength. W L₃-edge extended X-ray absorption fine structure (EXAFS) analyses for FeWO₄Cl and LiFeWO₄Cl show that the regular tetrahedron of the WO₄^2- group in FeWO₄Cl becomes distorted upon lithium intercalation.