A sodium ordering transition above room temperature in NaxCoO2 deduced by mechanical spectrum

The mechanical spectrum of Na0.66CoO2 above room temperature was measurement using the vibrating reed method at kilohertz frequency. A novel transition was observed around 427 K as indicated by temperature-dependent internal friction and resonance frequency. The characteristic transition temperature is defined as the start of a step increase of resonance frequency with the decrease of temperature, which is concurrent with the decrease of internal friction below a plateau. At the same time, this characteristic transition temperature shifts towards lower temperatures at a lower cooling rate, or higher heating rate during its respective thermodynamic processes. This unconventional behavior seems to be explained by critical slowing down at a second-order phase transition. Furthermore, sodium ordering is expected for this phase transition.