Giant thermal hysteresis of sound velocity and internal friction in a La$_{0.8}$Sr$_{0.2}$MnO$_3$ single crystal

Temperature dependences of the velocity of longitudinal sound $V_1$ and the internal friction $Q^{-1}$ are studied for a La$_{0.8}$Sr$_{0.2}$MnO$_3$ single crystal in the temperature range $5$–$350$ K. The latter includes the temperature of the structural phase transition $T_s\approx95\,$ K (from the $Pnma$ orthorhombic low-temperature phase to the $R\bar{3}c$ rhombohedral high-temperature one) and the Curie point $T_c=308\,$ K. Near the temperatures $T_s$ and $T_c$, the curves $V_1(T)$ and $Q^{-1}(T)$ exhibit pronounced singularities. Outside the vicinities of $T_s$ and $T_c$, the velocity of sound monotonically decreases with increasing temperature. A thermal hysteresis of giant width is observed in the aforementioned dependences. The hysteresis is attributed to the following mechanism: when the crystal under study is heated starting at temperatures $T<T_s$, some regions occupied by the $Pnma$ low-temperature phase are retained in the $R\bar{3}c$ matrix up to the temperature $T=350\,$ K.