Evidence of electron-phonon interaction in mixed-valence Na-ruthenate

We report a comprehensive investigation of the structural, electric transport, magnetic, and thermodynamic properties of Na2.7Ru4O9 and NaRu2O4 single crystals. The compounds are structurally different; Na2.7Ru4O9 crystalize in monoclinic (C 2/m) structure while NaRu2O4 crystalize in orthorhombic (P 21/ma) structure. We observed a first-order phase transition in the electrical resistivity at Tc = 335 K and Tc = 530 K for Na2.7Ru4O9 and NaRu2O4, respectively. The electrical resistivity data are then theoretically analysed within the framework of the classical electron–phonon model of resistivity, i.e. the Bloch–Grüneisen– Mott model. Detailed analysis of the electrical resistivity of both Ru-based metallic systems Na2.7Ru4O9 and NaRu2O4 suggests that the charge carrier density is strongly influenced by electron–phonon scattering and additionally inter-band electron scattering for Na2.7Ru4O9. The characteristic Debye temperature (Theta-R) is found to be 140 and 499 K for Na2.7Ru4O9 and NaRu2O4, respectively. Magnetic susceptibility Chi(T) = M/H curve shows diamagnetic behaviour for both the compounds and no magnetically ordered state was observed down to 1.9 K. The evidence for metal-like electronic contribution in the lowtemperature heat capacity was also observed for both the compounds. The electronic contribution to the specific heat (gamma) for Na2.7Ru4O9 and NaRu2O4 was determined to be 26.91 and 3.93 mJ/mol K2, respectively. The higher value of gamma for Na2.7Ru4O9 is much larger than the free electron value of 1 mJ/mol K2 Ru, indicates that Na2.7Ru4O9 belongs to the class of strongly correlated electron system.