Low thermal conductivity of the layered oxide (Na,Ca)Co2O4: Another example of a phonon glass and an electron crystal

The thermal conductivity of polycrystalline samples of (Na,Ca)Co2O4 is foun d to be unusually low, 20 mW/cmK at 280 K. On the assumption of the Wiedema nn-Franz law, the lattice thermal conductivity is estimated to be 18 mW/cm K at 280 K, and it does not change appreciably with the substitution of Ca for Na. A quantitative analysis has revealed that the phonon mean free path is comparable with the lattice parameters, where the point-defect scatteri ng plays an important role. Electronically the same samples show a metallic conduction down to 4.2 K, which strongly suggests that NaCo2O4 exhibits a glasslike poor thermal conduction together with a metal-like good electrica l conduction. The present study further suggests that a strongly correlated system with layered structure can act as a material of a phonon glass and an electron crystal.