“…Then the internal friction peak in as-prepared ␥-Na 0.66 CoO 2 is expected for sodium off-center hopping near oxygen vacancies. The atomic off-center hopping anelastic relaxation has been used to interpret the internal friction peaks in YBa 2 Cu 3 O 7−ı with the activation energy 0.16 and 0.19 eV [34,35].…”