Structural phase transitions in the LaMnO3+λ system

Structural phase transitions in the LaMnO_3+λ system are studied at various temperatures and with different Mn⁴⁺ ion concentrations. Two structural phase transitions are established. The O′-orthorhombic – O-orthorhombic transition (O′ O transition) is due to removing the cooperative Jahn-Teller distortions and occurs in the Mn⁴⁺ concentration range from 10 to 14% at room temperature. The temperature of this transition varies from 710 K for the stoichiometric LaMnO₃ to room temperature for LaMnO_3.07. The O-orthorhombic – rhombohedral transition (O R transition) is due to the change of the rotation axis of the undistorted MnO₆ octahedra from [100] and [110] to [11 1]. The temperature of this transition varies from 1010 K for LaMnO₃ to 230 K for LaMnO_3.135. The structural phase diagram of temperature vs composition is plotted, indicating the existence of regions of the phase with different structures.