ESR evidence for structural rearrangements occurring upon MoO3 reduction

Reduction of polycrystalline MoO₃ in the course of heat treatmentin vacuo has been studied by means of ESR. The formation of two different Mo(V) centers has been identified: center A withg_x = 1.946, g_y = 1.960, g_z = 1.868; and center B characterized byg_x = 1.943, g_y = 1.954, g_z = 1.878. Center B exhibited a well-resolved hyperfine structure resulting from naturally abundant odd isotopes of Mo:A_x = 3.2mT, A_y = 3.0mT, A_z = 7.0mT. Species A formed at an early stage of reduction has been identified as Mo(V) in rhombically distorted square pyramidal surrounding, whereas center B appearing in strongly reduced samples shows a distorted octahedral coordination. The results have been interpreted in terms of the crystallographic shear mechanism.