Symmetry breaking and unconventional charge ordering in single crystal Na$_{2.7}$Ru$_4$O$_9$

TL;DR

This study uncovers an unconventional charge ordering in Na$_{2.7}$Ru$_4$O$_9$ that occurs at room temperature, involving complex phase transitions and mixed valence states, while maintaining metallic conductivity.

Contribution

It presents the first detailed investigation of charge ordering phenomena in Na$_{2.7}$Ru$_4$O$_9$, revealing unique phase transitions and mixed valence states in a Ru-based compound.

Findings

Unconventional charge order at room temperature with super-lattice reflections.

Successive phase transitions at 365 K and 630 K.

Evidence of localized 4d Ru-electrons and mixed valence states.

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 ($\rho$), specific heat, susceptibility $\chi$ (\textit{T}), and single crystal x-ray diffraction (SC-XRD) that Na$_{2.7}$Ru$_4$O$_9$ with the monoclinic tunnel type lattice (space group $C$2/$m$) exhibits an unconventional CO at room temperature while retaining metallicity. The temperature-dependent SC-XRD results show successive phase transitions with super-lattice reflections at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) and \textbf{q}$_2$=(0, $\frac{1}{3}$, $\frac{1}{3}$) below $T_{\textrm{C2}}$ (365 K) and only at \textbf{q}$_1$=(0, $\frac{1}{2}$, 0) between $T_{\textrm{C2}}$ and $T_{\textrm{C1}}$ (630 K). We interpreted these as an evidence for the formation of an unconventional CO. It reveals a strong first-order phase transition in the electrical resistivity at $T_{\textrm{C2}}$ (cooling) = 345 K and $T_{\textrm{C2}}$ (heating) = 365 K. We argue that the origin of the phase transition is due to the localized 4$d$ Ru-electrons. The results of our finding reveal an unique example of Ru$^{3+}$/Ru$^{4+}$ mixed valance heavy \textit{d}$^4$ ions.