The mechanical spectrum of γ-Na0.66CoO2 below room temperature was measured using the vibrating reed method at kilohertz frequencies. In the cooling process, internal friction was less temperature dependent above 240 K and showed a continuous decrease with the decrease in temperature below 240 K in annealed γ-Na0.66CoO2 sample. A transition was suggested to occur around 240 K, which was proposed to be a freezing transition of the mobility of sodium atoms. A similar behavior was observed in as-prepared γ-Na0.66CoO2 sample. However, another internal friction peak appeared around 160 K. This internal friction peak was expected to be a thermally activated relaxation peak with some relationship to oxygen vacancies

Mechanical Spectrum and Oxygen Vacancies in γ-Na0.66CoO2. L.Zhang, X.N.Ying, Z.C.Xu: Journal of Alloys and Compounds, 2011, 509[13], 4622-5