Orbital Ordering and Vibronic States in Manganese Perovskites

Abstract

The structural, magnetic, and transport properties of LaMnO₃ reveal a progressive change from a static orbital ordering into (001) planes at temperatures T < T* ≈ 600 K to a 2D orbital disorder in the interval T* < T < T_JT ≈ 740 K and a 3D orbital disorder with partial disproportionation into Mn(II) + Mn(IV) for T > T_JT. The thermal conductivity, measured below 300 K, shows a strong phonon contribution contrary to previous reports made on oxidized samples. Ferromagnetic order in the insulator LaMn_0.5Ga_0.5O₃ confirms the existence of 3D ferromagnetic Mn(III)-O-Mn(III) vibronic superexchange interactions. Pressure on single-crystal samples was used as a variable to clarify the complex phase diagram of the La_1−xSr_xMnO₃ system in the compositional range 0.12 ≤ x ≤ 0.17. Three thermodynamically distinguishable ferromagnetic phases could be identified. Dynamic orbital disorder in vibronic phases and dynamic two-phase fluctuations are shown to suppress dramatically the thermal conductivity.