Mechanical spectrum and oxygen vacancies in γ-Na0.66CoO2

► Mechanical spectrum of γ-Na0.66CoO2 was measured in the range between liquid nitrogen and room temperatures. ► A freezing transition of sodium atoms around 240 K is indicated in γ-Na0.66CoO2. ► A relaxation peak related to oxygen vacancies was found near 160 K in as-prepared γ-Na0.66CoO2 sample.