Crystallographic features in lightly-doped La_{1 - x}Sr _xMnO₃

The crystallographic features of the simple perovskite oxides La _1-xSr_xMnO₃ with 0≤x≤0.08, which are known to be correlated-electron insulators, have been examined by X-ray powder diffraction and transmission electron microscopy. It was confirmed on the basis of the powder diffraction profiles obtained that the orthorhombic-to-monoclinic structural phase transition took place; for instance, at 500 K in the x=0.08 sample. The profiles also indicated that the structural transition accompanied the introduction of the Jahn-Teller distortion and the increase in both the orthorhombic distortion and the A-site-ion displacement. Based on the reciprocal lattices determined from electron diffraction patterns, further, it was found that these distortions and displacement never resulted in the change in the extinction rule of diffraction spots, and that the monoclinic distortion was actually involved in the low-temperature phase. That is, the structural phase transition can basically be regarded as a ferroelastic transition in terms of the lowering of the symmetry.