Single-crystal growth of NaxCo2O4 via a novel low-temperature flux method

NaxCo2O4 single-crystals have been grown using a novel low-temperature flux method. As compared to the formerly reported recipe, the growth temperature has been lowered almost by half to 550 °C via using pure Co as the Co source and the NaOH/NaCl admixture as the flux. The issue of significant loss of Na due to its high vapor pressure at elevated temperatures is much better controlled. Various growth conditions, such as the NaOH to NaCl ratio in the flux, the ratio of Co element to the flux, the firing temperature,the soaking time,the warming/cooling rates and the atmosphere control, have been optimized. The as-grown NaxCo2O4 single-crystals, in form of platelets with growth direction along the ab-plane and with a typical size of ∼ few mm on one edge, are found to be Na deficient (x∼0.3–0.8), which is consistent with the observed electrical resistivity, thermopower and magnetic susceptibility behavior. Despite the single-crystal nature, the directly measured in-plane thermal conductivity κ exhibited an amorphous-like temperature dependence; with the room temperature κ value is ∼7 W/m-K. In addition, the thermal stability and the associated surface micro-morphology changes have been studied by means of the differential scanning calorimetry–thermogravimetry analysis and scanning electron microscope. The results showed that the as-grown NaxCo2O4 single-crystals start losing Na above ∼900 °C.