Investigating Charge Order in Pb3Rh7O15

Abstract

Pb3Rh7O15 is a mixed-valent compound containing both Rh3+ and Rh4+ cations. This material undergoes a conductor-to-insulator transition at approximately 180 K. It is considered the first 4d transition metal compound to potentially exhibit a Verwey transition. Above the transition temperature, the Rh3+ and Rh4+ cations are expected to have an equal probability of occupying the same crystallographic site in the structure, resulting in conducting behaviour. Below this characteristic temperature, it is believed that these cations separate and arrange in a specific charge ordering scheme, causing the resistivity to increase abruptly. Despite the high-temperature structure of Pb3Rh7O15 being reported in 1978, its low-temperature structure remains unsolved. In this work, we investigate whether charge order is indeed the driving mechanism of this transition.

We begin with reviewing essential background theory on metal-to-insulator transitions and transition metal oxides. We then identify the experimental signatures of charge order observed in other Verwey compounds. After establishing previously reported characterizations of Pb3Rh7O15 , we present our own temperature-dependent comparisons of the average crystal structure obtained via single crystal X-ray diffraction (XRD) refinement, which we use for subsequent symmetry and bond valence sum (BVS) analyses. We then investigate the local structural environment of the Rh cations and their short-range correlations through the search for diffuse scattering, by means of reciprocal space viewing of single crystal diffraction measurements, as well as synchrotron-based techniques such as High Dynamic Range Reciprocal Space Mapping (HDRM) and 3D-∆ PDF analysis. Finally, we consider the electronic environment of the Rh cations at different temperatures using X-ray absorption spectroscopy (XAS) and FDMNES modelling. Across all analyses, the results consistently show no evidence of charge order.