X-ray study of structural phase transitions in nanocrystalline LaMnO3+δ perovskite

The nanocrystalline LaMnO3+delta perovskite was synthesized by the microwave-assisted glycothermal method and calcined at several temperatures up to 1500 degrees C. The prepared samples were examined by the X-ray powder diffraction with the aim to show that LaMnO3+delta exhibits the size-induced structural phase transitions. The as-received nanocrystals of LaMnO3+delta are of tetragonal, pseudo-cubic symmetry not known for bulk material. The samples calcined at temperatures 750-1100 degrees C have trigonal symmetry known from the high-temperature phase of LaMnO3 single crystal. The samples calcined from 1200 degrees C to 1500 degrees C have two phases: trigonal and orthorhombic. Thus, the observed phase sequence is inverted with respect to that of the bulk material, which is the characteristic of the size-induced mechanism of phase transitions in the nanocrystals. The critical crystallite sizes for both structural transitions were evaluated as 20 and 100nm.