Synthesis and physical properties of NaxCo2O4 single crystals

The thermopower, resistivity and thermal conductivity of NaCo₂O₄ single crystals which were grown by typical NaCl flux method were measured by our custom designed systems. A novel low temperature flux method where NaCl/NaOH was used as flux and metallic Co powders as Co source was developed to successfully synthesize Na-deficient Na_xCo₂O₄ crystals with size up to 6 mm at low temperature of 550°C. The different behavior in resistivity of the crystals grown via low temperature flux method revealed there exist two different types of crystals, one is metallic and another is semiconducting. Measurements on the compact pellet of crystals which were pressed under pressure and temperature revealed the semiconducting resistivity behavior, where energy gap E_g was found to be 0.09-0.3 eV. The heat capacity and Debye temperature of single crystals are also discussed.