Reversible low-temperature transition in LaMnO3+δ

The evolution with a decrease in temperature of the basic rhombohedral structure of the LaMnO3+δ compound having a specified constant oxygen concentration is considered. The X-ray diffraction analysis shows that a transition from rhombohedral to monoclinic phase begins at a temperature close to 200 K. The important feature of the revealed low-temperature phase transition is its reversibility with heating to room temperature. A deformation mechanism of structural phase transitions based on uniform distortions of a perovskite block with a doubled lattice parameter is proposed. The results of the analysis of the symmetry of atomic positions are in favor of two-step routes of phase transitions through dilatation and twinning.