21. Pseudo-Commensurate GdBaCo₂O_5+δ and Its Phase Transition at Elevated Temperatures

An in-situ single-crystal X-ray diffraction study on tetragonal GdBaCo₂O_5+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.