Giant thermal hysteresis of sound velocity and internal friction in a La0.8Sr0.2MnO3 single crystal

Temperature dependences of the velocity of longitudinal sound V ₁ and the internal friction Q ^?1 are studied for a La_0.8Sr_0.2MnO₃ single crystal in the temperature range 5–350 K. The latter includes the temperature of the structural phase transition T _s ≈95 K (from the Pnma orthorhombic low-temperature phase to the $Rbar 3c$ 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 $Rbar 3c$ matrix up to the temperature T=350 K.