Metal-insulator transition in Ba₃Fe_1−xRu_2+xO₉ : Interplay between site disorder, chemical percolation, and electronic structure

Abstract

Perovskites containing barium metal at the A site often take up unusual hexagonal structures having more than one type of possible sites for the B cation to occupy. This opens up various different B−B- or B-O-B-type connectivities and consequent physical properties which are naturally missing in cubic perovskites. BaRuO₃ is one such system where doping of Ru (4d⁴) by other transition metals (Mⁿ⁺) creates similar conditions, giving rise to various M-Ru interactions. Interestingly, the 6H hexagonal structure of doped barium ruthenate triple perovskite (Ba₃MRu₂O₉) seems to possess some internal checks because within the structure M ion always occupies the 2a site and Ru goes to the 4f site, allowing only M-O-Ru 180° and Ru-O-Ru 90° interactions to occur. The only exception is observed in the case of the Fe dopant, which allows us to study almost the full Ba₃Fe_1−xRu_2+xO₉ series of compounds with wide ranges of x because here Fe ions have the ability to freely go to the 4f sites and Ru readily takes up the 2a positions. Therefore, here one has the opportunity to probe the evolution of electronic and magnetic properties as a function of doping by going from BaRuO₃ (paramagnetic metal) to BaFeO₃ (ferromagnetic insulator). Our detailed experimental and theoretical results show that the series does exhibit a percolative metal-insulator transition with an accompanying but not coincidental magnetic transition as a function of x.