Incommensurate structure of GdBaCo2O5+δ (δ∼0.38)

The incommensurate structure of the GdBaCo2O5+δ (δ∼0.38) paramagnetic phase was studied using the single-crystal X-ray diffraction. The basic structure of the compound is a double-layered perovskite with an alternating layer sequence of [GdOδ]–[CoO2]–[BaO]–[CoO2] along the c axis. The crystal belongs to the five-dimensional superspace group P4/mmm(α00)0000(0α0)0000, α=0.3368(1), with a 1×1×2-type tetragonal fundamental unit cell of a=3.8934(1) Å and c=7.5267(1) Å. The structure was refined to R=0.028 for all the observed 2216 reflections with I>3σ(I), including 331 main reflections (R=0.017), 1039 observed first-order satellites (R=0.033), and 846 observed second-order satellites (R=0.045). The crystal has oxygen deficiency that occurs only in the [GdOδ] layer; however, it causes many positional modulations of the constituent atoms throughout the crystal in association with a valence fluctuation of Co between the divalent and trivalent states. Because the value of α was very close to 1/3, the structure was also investigated using both the commensurately modulated approach and the conventional three-dimensional approach assuming a 3×3×2 supercell of P4/mmm symmetry. These approaches successfully reproduced a prime structure of the compound that consists of intersecting CoO5 pyramidal arrays parallel to a or b axes. The bond valence sum and the charge neutrality principle suggested that the divalent and trivalent cobalt cations are distributed in an ordered way. The incommensurate approach also indicated the possible presence of a local disorder having a structural similarity with that of a high-temperature modification.