Synthesis and structural analysis of Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7}, a new pyrochlore solid solution
This paper presents a study of the synthesis and structural properties of the new pyrochlore-type Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} series. Ten compositions with 0.0{<=}y{<=}0.9 were prepared by solid-state reaction with thermal treatments at 873, 1073 and 1323 K under atmospheric pressure conditions. Structural refinements from X-ray powder diffraction data by the Rietveld method show that all compounds of the Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} solid solution crystallize in a {alpha}-pyrochlore structure. The main structural difference when bismuth is substituted by strontium concerns the x position of the O1 (x, (1/8), (1/8)). This substitution significantly increases the Bi/Sr-O1 distance and diminishes the Ir-O1 distance; this implies that the Ir-O1-Ir bond angle increases. With the Sr substitution, the IrO{sub 6} local configuration goes from a flattened trigonal antiprism, y<0.5, to an elongated one, y>0.5, passing through an octahedral array, y{approx}0.5. The electrical consequences of these structural changes observed in this system are qualitatively explained with electronic structure calculations, this behavior agrees very well with those observed in other pyrochlore systems A{sub 2}M{sub 2}O{sub 7} (A=rare earth cations or Tl{sup +}, Pb{sup 2+}, or Bi{sup 3+}, and M=Ru or Ir). -- Graphical abstract: Electrical behavior of the Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} solid solution. Display Omitted Highlights: {yields} Bi{sub 2-y}Sr{sub y}Ir{sub 2}O{sub 7} crystallizes in the {alpha}-pyrochlore structure. {yields} Local geometry of Ir goes from a flattened trigonal antiprism to an elongated one. {yields} Electrical resistivity increases with the Sr content in the solid solution. {yields} Electronic structure analysis relates resistivity with Sr disorder and bond strength.