Uncorrelated Bi off-centering and the insulator-to-metal transition in ruthenium A2Ru2 O7 pyrochlores

The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (Pr1-xBix)2Ru2O7. Prior work has established site disorder by the Bi3+ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state