An extended X-ray absorption fine structure (EXAFS) study of mixed molybdenum–tungsten M3 clusters in Zn2Mo3–xWxO8(x= 1, 1.5 or 2)

Abstract

Molybdenum K-edge and tungsten L_III-edge extended X-ray absorption fine structure spectroscopic (EXAFS) studies have been carried out on the mixed molybdenum–tungsten compounds Zn₂Mo_3–xW_xO₈ to investigate the metal–metal bonding in these materials and to determine whether there is any tendency to form homo- or hetero-metallic M₃ clusters. There is no local ordering in the metal–metal bonded M₃ triangles. The Mo–Mo, Mo–W and W–W distances are all 2.51 ± 0.02 Å. Molybdenum–oxygen distances in the M₃O₁₃ cluster refine to values in excellent agreement with those in the parent compound, Zn₂Mo₃O₈, and give bond valence sums for molybdenum close to four. The co-ordination geometry around tungsten is almost identical to that around molybdenum. The very small increase in. W–O bond lengths relative to Mo–O found in Zn₂MoW₂O₈ is consistent with the small increase in the unit-cell parameters between Zn₂Mo₃O₈ and Zn₂MoW₂O₈.