Incomplete structural phase transition and lattice destruction in Sr_1-xLa_xTiO_3+δ

In situ observation of the R3c-to-Pbnm structural phase transition in Sr_0.5La_0.5TiO_3+δ has been made mainly by transmission electron microscopy to understand features of the transition in samples with relatively large nonstoichiometry of more than δ=0.05. Instead of the appearance of the superlattice reflection spot exhibiting the phase transition, only the diffuse maximum appears at q = 1/2〈110〉 and q = 1/2〈100〉 in the simple cubic reciprocal lattice below a transition temperature T_s of about 190 K. The subsequent aging below T_s also results in the local lattice destruction. These features found in the samples with large nonstoichiometry should be caused by both the nonstoichiometry and the strain field induced by the change in the crystal system in the transition.