Pseudo-Commensurate GdBaCo2O5+δ and Its Phase Transition at Elevated Temperatures
An in-situ single-crystal X-ray diffraction study on tetragonal GdBaCo2O5+δ with δ∼0.38 revealed that the crystal is pseudo-commensurate at room temperature with the magnitudes of the modulation vectors q 1 and q 2 parallel to the basal axes increasing gradually from the nearly commensurate value close to 1/3 upon heating. The basic structure of the compound is a double-layered perovskite type, having an alternating layer sequence [GdO δ ]–[CoO2]–[BaO]–[CoO2] along the c axis. The oxygen deficiency of the crystal occurs only in the [GdO δ ] layer, though it causes many positional modulations of constituent atoms in association with the valence fluctuation of Co cations between +2 and +3. Because of its pseudo-commensurate nature, the room temperature structure was also investigated by the commensurately-modulated approach as well as the conventional three-dimensional ones assuming a 3×3×2 supercell of the P4/mmm symmetry. These approaches successfully reproduced a prime structure of the compound, consisting of intersecting CoO5 pyramidal arrays parallel to a and b axes. The incommensurate approach, on the other hand, also suggested a presence of a local disorder having a structural similarity with the high-temperature modification.
KeywordsGadolinium barium cobaltate GdBaCo2O5+δ Single-crystal X-ray diffraction Modulated structure
The authors are grateful to Dr Vaclav Petricek, Institute of Physics, Academy of Science, Czech Republic, for his valuable comments and calculations, and Dr. Terutoshi Sakakura, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan, for supplying us a Python script for data conversion. This work was supported by the Grant-in-Aids for Scientific Research No. 22360272 from the Japan Society for the Promotion of Science.
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