Kinetics of phase transitions and amplitude dependences of Q−1 in Na0.875Li0.125NbO3

•The fluctuation mechanism of the internal friction near phase transitions in the Na0.875Li0.125NbO3 lead free ferroelectric.•Ferroelectric domains contribution to attenuation of low-frequency sound.•The amplitude dependences of internal friction.•Friedel's dislocation model for amplitude dependences of internal friction. The temperature and amplitude dependences of internal friction Q−1 and shear modulus G for the Na0.875Li0.125NbO3 solid solution were studied in the temperature range of 300–700 K. It was found that the material undergoes two structural phase transitions near temperatures T1 ≈ 620 K and T2 ≈ 559 K. Both low-temperature phases are ferroelectric one, whereas the phase above the temperature T1 is paraelectric one. It is shown that the main contribution to mechanical losses below T1 is related to the motion of domain walls. The Q−1 maxima observed near temperatures T1 and T2 are satisfactory described within the framework of the low-frequency fluctuation mechanism of internal friction. The amplitude dependences of Q−1 are in agreement with the Friedel dislocation model.