Incommensurate structure of GdBaCo2O5 (δ∼0.38)

Hydration, oxidation, and reduction of GdBaCo2O5.5from first-principles

“…Distortions from the Pmmm space group, associated with appearance of a superstructure below the metal-insulator transition, have already been reported in GBCO, 39,54,56 and also in reduced GBCO. 55 Although the precise nature of the distortion remains unclear, the present calculation provides a candidate structure that could be compared to experiments. In this low-energy structure of GBCO, the Co ions are still found in the AFM3-HS state, but neither other AFM order nor spin state conguration has been tested.…”

Structural Evolution of GdBaCo2O5+δ (δ = 7/18) at Elevated Temperatures

“…Although the modulations are functions of the internalspace coordinates t and u in (3 + 2) dimensions, 36 the displacements are plotted at u = 0, because the extrema of the Gd1 and O3 displacements exist along the line at u = 0. 16 The maximum displacements are ∼0.2 Å for Gd1 and ∼0.4 Å for O3 along the a axis. These large displacements partly explain the difficulty in determining atomic positions in the [GdO δ ] layer in the (3 + 0)-dimensional analysis (see Section 3.3).…”

“…The 3 × 3 × 2 supercell model for the γ phase, however, suffered from an apparent disorder in Gd atom positions, as first encountered for Y in YBaCo 2 O 5+δ (δ ≈ 0.44) by Khalyavin et al 14 This problem essentially stems from the incommensurate nature of the γ phase, with atom positions being modulated from the basic positions by sinusoidal functions. 16 In the present analysis, the problem was avoided by introducing the anharmonic approximation for ADP up to the fifth order terms for Gd1 and Gd2. Harmonic ADPs were used for the other metal atoms and the isotropic ADPs for O.…”

“…The occupational modulation of O3 is also plotted in Figure 10b along the line t = u since the extrema occur along this line. 16 The O3-Occ varies over the full range while the overall average has been constrained at 7/18. In order to avoid confusion, it should be noted that this value can only be obtained by taking an average of all Occ data in the twodimensional range 0 ≤ t, u ≤ 1 and not only the data along the line t = u.…”

“…14 Recently, Asaka et al 15 used transmission electron microscopy to reveal that the γ phase of GdBaCo 2 O 5+δ (δ < 1/2) is actually incommensurate and undergoes a reversible phase transition into the 1 × 2 × 2 superstructure at ∼380 K. The changes in the modulation vectors with temperature were affected by the applied magnetic field under the electron microscope. A following single-crystal X-ray diffraction study 16 succeeded in solving the modulated structure using a (3 + 2)dimensional superspace approach. The γ phase thus determined had a unique feature: Co ions were essentially charge-ordered in a ratio of Co 3+ :Co 2+ = 8:1, resulting in an ideal composition GdBa [Co 3+ 8/9 Co 2+ 1/9 ] 2 O 5+δ (δ = 7/18) in the 3 × 3 × 2 supercell model.…”