Crystallographic features of La_1-xSr_xMO₃ with M = Cr and Fe in the vicinity of the R3̄c/Pbnm phase boundary

Crystallographic features of La_1-xSr_xMO₃ with M=Cr and Fe in the vicinity of the R3̄c/Pbnm phase boundary have been examined mainly at room temperature by transmission electron microscopy. When the Sr content x increases from x=0, the change in crystal structure from Pbnm to R3̄c was confirmed to occur at room temperature in La_1-xSr_xCrO₃ around x=0.2. In La_1-xSr_xFeO₃, on the other hand, the incommensurate phase was found to exist between the Pbnm and the R3̄c phases. The analysis of the electron diffraction patterns obtained in the work indicated that the normal structure of the incommensurate structure and the direction of the modulation could, respectively, be identified as the Pbnm structure and one of the 〈323〉 directions. It was also suggested that the rotational displacement of the oxygen octahedra involved in the Pbnm structure should be modulated in the incommensurate structure.