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 LIII-edge extended X-ray absorption fine structure spectroscopic (EXAFS) studies have been carried out on the mixed molybdenum–tungsten compounds Zn2Mo3–xWxO8 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.