Cobalt valence and crystal structure of La_0.5Sr_0.5CoO_2.25

Abstract

Oxygen vacancies play a key role in determining the functionality and performance of many oxide smart materials. In this paper, the crystal structure of an anion-deficient perovskite-type orthorhombic (with a ≈ b) phase La_0.5Sr_0.5CoO_2.25 (or La₈Sr₈Co₁₆)₃₆) has been determined for the first time with the combined use of energy-dispersive X-ray spectroscopy, electron-energy-loss spectroscopy, high-resolution transmission electron microscopy and electron diffraction. The unit cell is made of two types of fundamental module, and it is composed of a total of eight modules. Each module is a c-axis stacking of the anion-deficient SrCoO_3-z and LaCoO_3-δ basic perovskite cells. The unit cell preserves the characteristics of the perovskite framework, and it is a superstructure induced by O vacancies. This study illustrates the correlation of anion deficiency with the valence state of Co, and it also proves that the O atom positions can be determined using the combination of transmission electron microscopy and associated techniques. This is likely to be an effective method for probing the microstructure of perovskite-type functional materials.