Disorder–order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La2/3Sr1/3MnO3 ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La2/3Sr1/3MnO3 phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La–Sr ordered structure with space group P3¯m1, which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.