Orbital ordering and vibronic states in manganese perovskites

摘要

The structural, magnetic, and transport properties of LaMnO{sub}3 reveal a progressive change from a static orbital ordering into (001) planes at temperatures T ＜ T{sup}* ≈ 600 K to a 2D orbital disorder in the interval T{sup}* ＜T＜ T{sub}(JT) = 740 K and a 3D orbital disorder with partial disproportionation into Mn(II) + Mn(IV) for T ＞ T{sub}(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{sub}0.5Ga{sub}0.5O{sub}3 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{sub}(1-x)Sr{sub}xMnO{sub}3 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.