Microdomain texture and oxygen excess in the calcium-lanthanum ferrite: Ca2LaFe3O8

The analysis by TEM and electron diffraction of the anion-deficient perovskite Ca₂LaFe₃O₈ confirms the model previously proposed by J. C. Grenier et al. (Mater. Res. Bull.11, 1219 (1976)) with a structure intermediate between perovskite and brownmillerite. The unit cell parameters are ～√2a_c, 3a_c, √2a_c (where a_c is the cubic perovskite unit cell parameter). However, the unit cell is sometimes doubled along the b axis. When the sample is treated in air at temperatures around 1400°C, an oxidation process is observed and the unit cell becomes cubic (a_c = 3.848(3) Å). Nevertheless, electron diffraction investigations suggest the existence of a much more complex situation in which three-dimensional microdomains intergrow within one crystal. Each of these microdomains appears to have a structure clearly related to the low-temperature sample, but the superstructure is randomly found along each of the three cubic subcell directions (i.e., the unit cell √2a_c, √2a_c, 3a_c alternates randomly with 3a_c, √2a_c, √2a, and with √2a_c, 3a_c, √2a_c). High-resolution electron microscopy allows one to ascertain this microdomain texture of the real crystal.