Phase transition from Pnma I to Pnma II in the 57Fe-doped LaMnO3+δ compound

The structural phase transition from Pnma I to Pnma II in the 3% 57Fe-doped lanthanum manganite LaMnO3+δ has been investigated under heat treatment. It has been shown that the phase transition in the LaMnO3+δ compound goes through the formation of the intermediate phase Pnma II* under vacuum annealing at T = 650 °C (from 2 to 22 h). The transition from one phase to another, Pnma I → Pnma II* → Pnma II, occurs virtually by jump. It follows from the Mössbauer investigations that two types of environments with stronger and weaker distortion exist in the Pnma II* and Pnma II phases in contrast to Pnma I and R3¯c, in which one type of environment takes place only.