Structural and electrical properties of $Bi_{2-x}M_xIr_2O_7$ ($M$= $Na^+$, $Ca^{2+}$ and $Ce^{4+}$ ) pyrochlores

In $\alpha$-pyrochlores, $A_2B_2O_7$, it is possible to appreciate different magnetic couplings depending on A and B spin-arrangement. In order to investigate how these magnetic and electrical properties are related with the iridium oxidation-state, we investigate the $Bi_{2-x}M_xIr_2O_7$ system ($M$ = $Na^+$, $Ca^{2+}$ and $Ce^{4+}$). Polycrystalline samples have been synthesized by the conventional solid-state reaction method. Single-phase polycrystalline samples were confirmed by $X$-ray diffraction patterns. Structural characterizations were performed by $X$- ray Rietveld refinement of powders and the temperature dependence of the electrical resistivity was studied in the 10- 300 K range. The samples showed a metallic-type behavior that depended on the Iridium oxidation-state. This behavior could be explained on the basis of the hole/electron doping when we introduce some different $Bi^{3+}$-substituent metals in order to change the iridium electronic state. In this way, $Ce^{4+}$- doping will produce a change from $Ir^{4+}$ ($t_{2g}^5e_g^0$, electronic state in octahedral coordination) to $Ir^{3+}$ ($t_ {2g}^6e_g^0$)