Symmetry breaking and unconventional charge ordering in single crystal Na2.7Ru4 O9

Abstract

The interplay of charge, spin, and lattice degrees of freedom in matter leads to various forms of ordered states through phase transitions. An important subclass of these phenomena of complex materials is charge ordering (CO), mainly driven by mixed-valence states. We discovered by combining the results of electrical resistivity (ρ), specific heat, susceptibility (χ), and single crystal x-ray diffraction (SC-XRD) that Na2.7Ru4O9 with the monoclinic tunnel type lattice (space group C2/m) exhibits an unconventional CO at room temperature while retaining metallicity. The temperature-dependent SC-XRD results show successive phase transitions with superlattice reflections at q1=(0,12,0) and q2=(0,13,13) below TC2 (365 K) and only at q1=(0,12,0) between TC2 and TC1 (630 K). We interpreted these as evidence for the formation of an unconventional CO. It reveals a strong first-order phase transition in the electrical resistivity at TC2 (cooling) = 345 K and TC2 (heating) = 365 K. We argue that the origin of the phase transition is due to the localized 4d Ru electrons. The results of our finding reveal an unique example of Ru3+/Ru4+ mixed valance heavy d4 ions. © 2018 American Physical Society