XAFS analysis of the low symmetry octahedral molybdenum and tungsten oxides

Comparative XAFS analysis of the main isostructural crystalline phases of W(Mo)O₃ oxides and W(Mo)O₃·nH₂O hydrates has been done at the W L₃ and Mo K edges. These compounds have (1) 3D perovskite-type (PT) structure (m-WO₃); (2) 2D PT structure (WO₃·H₂O and MoO₃·nH₂O (n = 1,2)); (3) 2D hexagonal-type structure (WO₃·1/3H₂O) and (4) 2D double layered structure (α-MoO₃). The detailed analysis of X-ray absorption fine structure (XAFS) in the low-symmetry structures shows that in order to simulate the experimental spectrum, it is sufficient to take into account single-scattering contributions in the range from 1.7 to ∼ 5.0 A ̊ and a number of multiple-scattering paths from nearly linear atomic chains in the first and second shells. The results obtained by XAFS are only in partial agreement with known X-ray diffraction data (XRD). The strong deviation of the short-range order, detected by XAFS, from the one, given by XRD, has been found in W(Mo)O₃·nH₂O.