Structure and reactivity of the oxyanions of transition metals. Part XVII. A study of NbO42– in calcium molybdate by electron spin resonance

Abstract

A paramagnetic centre, thought to be NbO₄^2–, has been obtained by γ-irradiation of a single crystal of CaMoO₄ : Sm: Nb at 77°K. At 77°K it appears in four magnetically distinct sites and the e.s.r. spectra can be described by an anisotropic spin Hamiltonian with principal values of the g- and A-tensors given by g_xx= 2·0061, g_yy= 2·0146, g_zz= 2·0480, and A_xx=–25·5, A_yy=–28·5, A_zz=–30·5 gauss. The two tensors are approximately spatially coincident with g_zz lying close to one of the Nb–O bonds. Also, isotropic super-hyperfine interaction with one neighbouring MoO₄^2– anion was observed with A′_iso= 2·2 G. At 195°K there is only one distinct site and the spin Hamiltonian is axially symmetric having g_∥= 2·0208, g_⊥= 2·0250 and A_∥=–27·9, A_⊥=–27·0 G, the unique direction lying along the crystallographic c axis. The magnetic parameters at 77°K are best reproduced in terms of a model in which the electron deficiency of NbO₄^2– is localised almost entirely in an oxygen π-orbital. The super-hyperfine coupling to Mo is interpreted in terms of weak bonding between NbO₄^2– and one of the four nearest-neighbour MoO₄^2– ions, and the results at 195°K are then explained in terms of a rapid jumping between the four equivalent sites.