Investigating Charge Order and the Metal-Insulator Transition in Pb₃Rh₇O₁₅.

Pb₃Rh₇O₁₅ is a mixed-valent compound containing both Rh³⁺ and Rh⁴⁺ cations. This material undergoes a metal-to-insulator transition at approximately 180 K. It is theorized to be a candidate for the first 4d transition metal oxide presenting a Verwey transition. Above the transition temperature, the Rh³⁺ and Rh⁴⁺ cations have an equal probability of occupying the same Rh site in the crystal structure, resulting in metallic behaviour in Pb₃Rh₇O₁₅. Below this characteristic temperature, it has been proposed that these cations separate and arrange in a specific charge ordering scheme, causing the compound to behave as an insulator. However, despite the high-temperature structure of Pb₃Rh₇O₁₅ being reported by Boonstra and Mutsaers in 1978, the low-temperature structure remains unsolved. In this work, we investigate whether charge order is indeed the driving mechanism of the metal-insulator transition in this system. We perform temperature-dependent comparisons of the average crystal structure, local structural environment, and electronic environment of the Rh cations using a combination of single-crystal x-ray diffraction, high dynamic range reciprocal space mapping, 3D-△ PDF analysis, and x-ray absorption spectroscopy.