1. Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, New Jersey 08540, USA
2. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
3. NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; and Department of Materials and Nuclear Engineering, University of Maryland, College Park, Maryland 20742, USA
4. Lucent Technologies, Murray Hill, New Jersey 07974, USA
5. Present address: Condensed Matter and Thermal Physics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.

Correspondence to: R. J. Cava¹ Correspondence and requests for materials should be addressed to R.J.C. (e-mail: Email: rcava@princeton.edu).

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal–oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO₃ to low-temperature superconductivity in Sr₂RuO₄ (refs 1, 2). Here we show that a ruthenate in a different structural family, La₄Ru₆O₁₉, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La₄Ru₆O₁₉ have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu_6-xAu_x—a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru–Ru bonding and delocalized states derived from Ru–O hybridization.