ELECTRONIC-PROPERTIES AND PHASE-TRANSFORMATIONS IN COMOO4 AND NIMOO4 - XANES AND TIME-RESOLVED SYNCHROTRON XRD STUDIES

The thermal stability of a series of cobalt and nickel molybdates (AMo O4 . nH(2)O, alpha-AMo(4), and beta-AMoO(4); A = Co or Ni) was examine d using synchrotron-based time-resolved X-ray powder diffraction (XRD) . The results of X-ray absorption near-edge spectroscopy (XANES) indic ate that the Co and Ni atoms are in octahedral sites in all these comp ounds, while the coordination of Mo varies from octahedral in the alph a-phases to tetrahedral in the beta-phases and hydrates. Upon heating of AMoO(4) . nH(2)O, evolution-of gaseous water was seen at two differ ent temperature ranges: 100-200 degrees C for reversibly bound H2O; 20 0-400 degrees C for H2O from the crystal structure. The results of tim e-resolved XRD show a direct transformation of the hydrates into the b eta-AMoO(4) compounds (following a kinetics of first order) without an y intermediate phase. This is probably facilitated by the similarities that AMoO4 . nH(2)O and beta-AMoO(4) have in their structural and ele ctronic properties. The XRD experiments show that the alpha-AMoO(4)--> beta-AMoO(4) transitions occur at much higher temperatures than the hy drate --> beta-AMoO(4) transformations (Delta T approximate to 150 deg rees C in CoMoO4 and 280 degrees C in NiMoO4). The activation energy f or the alpha-NiMoO4 --> beta-NiMoO4 transition is similar to 40 kcal/m ol larger than that for the NiMoO4 . nH(2)O --> beta-NiMoO4 + nH(2)O r eaction. The larger activation energy reflects the change in the coord ination of Mo (O-h --> T-d) that occurs during the alpha --> beta tran sition. The K- and L-edges of Co in XANES spectra indicate that there are no big variations in the electronic properties of this metal when comparing CoMoO4 . nH(2)O, alpha-CoMoO4, and beta-CoMoO4. The same is valid for the electronic properties of Ni in the nickel molybdates. In contrast, the L-2,L-3-edges of Mo show large changes in the splitting of the Mo 4d orbitals as the coordination of this metal varies from o ctahedral (alpha-phases) to tetrahedral (beta-phases and hydrates). Th e features near the threshold in the O K-edge spectra track very well the splitting of the Mo 4d orbitals in tetrahedral and,octahedral fiel ds and can be very useful for probing the local symmetry of Mo atoms i n molybdenum oxides.