Direct Evidence of Anomalous Peierls Transition-Induced Charge Density Wave Order at Room Temperature in Metallic NaRu2O4.

In the field of quantum materials, understanding anomalous behavior under charge degrees of freedom through bond formation is of fundamental importance, with two key concepts: Dimerization and charge order at different cation sites. The coexistence of both dimerization and charge ordering is unusually found in NaRu2O4, even in its metallic state at room temperature. Our work unveils the origin of the interplay of these effects within metallic single-crystalline NaRu2O4. Employing advanced transmission electron microscopy techniques, we probe the lattice order of NaRu2O4 as a function of temperature and provide direct microscopic evidence of a Peierls-type transition. This transition is accompanied by a pronounced dimerization of the ruthenium chains, resulting in a distinctive twofold superstructure along the b axis below the critical transition temperature of ∼535 K, coinciding with a charge order. In situ heating experiments confirm the reversibility of this first-order phase transition, and periodic lattice displacement maps depict atomic-scale displacements linked to dimerization.