Crystallographic and magnetic properties of Pb_2-xBi_xIr₂O_7-δ (0≤x≤2)

The title series of compounds have been synthesized by solid state reaction and characterized by x-ray diffraction (PXD), neutron powder diffraction (PND), second harmonic generation (SHG), x-ray absorption spectroscopy (XAS) and magnetization measurements. The crystal structures have been refined from PND data in the centrosymmetric space group Fd3¯m with pyrochlore structure. No sign of a break of the centrosymmetry has been observed by SHG and no change of the structure to a polar space group in any component of the series, as it was previously reported for Pb₂Ir₂O_7-δ with non-centrosymmetric F4¯3 m space group. We have determined by PND that the oxidation state of Ir is slightly decreasing from 5⁺-to-4⁺ in the Pb-rich to almost 4⁺ in the Bi-rich samples. This evolution is confirmed by XAS and also it explains the progression of the crystallographic parameters. All the samples are paramagnetic in the temperature range measured and the magnitude of the effective magnetic moment is enhanced with Bi content, correlated with the enhancement of Ir⁴⁺ compared to Ir⁵⁺; suggesting Ir⁵⁺ to be present in a trigonal antiprism crystal field splitting and therefore Ir⁴⁺ as the only magnetic cation.