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 (AMoO4·nH2O, α-AMoO4, and β-AMoO4; A = Co or Ni) was examined using synchrotron-based time-resolved X-ray powder diffraction (XRD). The results of X-ray absorption near-edge spectroscopy (XANES) indicate that the Co and Ni atoms are in octahedral sites in all these compounds, while the coordination of Mo varies from octahedral in the α-phases to tetrahedral in the β-phases and hydrates. Upon heating of AMoO4·nH2O, evolution of gaseous water was seen at two different temperature ranges:  100−200 °C for reversibly bound H2O; 200−400 °C for H2O from the crystal structure. The results of time-resolved XRD show a direct transformation of the hydrates into the β-AMoO4 compounds (following a kinetics of first order) without any intermediate phase. This is probably facilitated by the similarities that AMoO4·nH2O and β-AMoO4 have in their structural and electronic properties. The XRD experiments show that the α-AMoO4 → β-AMoO4 transitions occur at m...