The crystal structure and twinning behavior of ferric molybdate, Fe2(MoO4)3

Fe2(MoO4)3 is monoclinic, space group P21a with a = 15.707, b = 9.231, c = 18.204 Å and β = 125.25°. The structure was solved by the direct method and Fourier technique, and refined by least squares to an R = 0.028. The structure contains isolated MoO4 tetrahedra and FeO6 octahedra, which share corners. The rather open structure shows apparent flexibility and an ease to twinning. The twinning behavior is related to the pseudosymmetry and the ferroelastic transition.