Crystal structure and electrical properties of the Bi$_{2-y}$Sr$_{y}$Ir$_{2}$O$_{7}$ $\alpha$-pyrochlore solid solution

In this work we report the synthesis and crystal structure of the Bi$_{2-y}$Sr$_{y}$Ir$_{2}$O$_{7}$. From structural Rietveld refinements we show that in this system the local geometry of the IrO$_{6}$ passes from a trigonal antiprism (y$<$0.4); a regular octahedron (y=0.5), reaching a new trigonal antiprism at the end compositions (y$>$0.5). Experimentally, this is a metallic system with a conductivity that decreases as a function of the Sr content in the (10-300 K) low temperature range. By means of electronic structure calculations, using WIEN2k to study Bi$_{2}$Ir$_{2}$O$_{7}$ and two hypothetical compounds, BiSrIr$_{2}$O$_{7}$ and h-BiSrIr$_{2}$O$_{7}$, we show that a) the main contribution to conductivity come from the shift of the oxygen towards the Ir atoms; b) the lattice imperfections (random occupation of Sr) and lattice vibrations are responsible for the drop of the electrical conductivity, and c) the IrO$_{6}$ local geometry (and crystal field configuration, t$_{2g}^{5}$ e$_{g}^{0}$/e$_{g}^{4}$a$_{1g}^{1}$b$_{2g}$b$_{1g}$ change), this last one does not seem to affect the electrical conductivity.