Ab initio structure determination and Rietveld refinement of Bi 10Mo 3O 24 the member n=3 of the Bi 2n+4Mo nO 6(n+1) series

Abstract

Bi 2O 3–MoO 3 system shows a large panoply of phases depending on Bi/Mo ratio, among them, the low temperature phases of the homologous series Bi 2( n +2)Mo n O 6( n +1) with n=3, 4, 5 and 6. They exhibit, alike most of the phases of this system, strong fluorite sub-network. Nevertheless, a multitechnique approach has been followed in order to solve the crystal structure of the n=3 member, i.e. Bi 10Mo 3O 24. From ab initio indexing X-ray powder pattern cell parameters were derived. It belongs to the monoclinic system, space group C2, with cell parameters: a=23.7282(2) Å, b=5.64906(6) Å, c=8.68173(9) Å, β=95.8668(7)° with Z=2. The matrix relating this cell with the fluorite one is 4 0 1/0 1 0/ - 1 2 0 3 2 and a cationic localization was derived. HRTEM allowed the cationic Bi and Mo order to be modified and specified, as well as to build up a full structural ab initio model on the basis of crystal chemistry considerations. Simultaneous Rietveld refinement of multipattern X-ray and neutron powder diffraction data taking advantage of the neutron scattering length for O location have been performed. The goodness of the model was ascertained by low reliability factors, weighted R b=4.97% and R f=3.21%. This complex Bi 10Mo 3O 24 structure, with 5Bi, 2Mo and 13O in different crystallographic positions of the asymmetric unit, shows good agreement between observed and calculated patterns within the data resolution. Moreover, the determination of this structure sets the basis for the crystallographic characterization of the complete family Bi 2( n +2)Mo n O 6( n +1), whose guidelines are also evidenced in this paper.