X-ray Diffraction Study of Superstructure in GdBaCo2O5.5
  A single crystal of GdBaCo2O5.47(2) has been studied by means of X-ray diffraction. Appearance of superstructure reflections at T = 341.5(7) K gives an evidence of continuous transition to the phase with unit cell doubled along the shortest edge a1. Critical exponent for the order parameter is found to be beta=0.33(1). The superstructure reflections are about 2-4 orders of magnitude weaker than the basic ones. Their systematic extinction indicates the crystal symmetry change from Pmmm to Pmma. The integrated intensities allow to calculate displacements of atoms from the positions in the high-temperature phase. The cobalt-ligand distances in the ordered phase are discussed in terms of the spin-state/orbital ordering of Co3+ ions. 
Yu. P. Chernenkov, V. P. Plakhty, V. I. Fedorov, S. N. Barilo, S. V.   Shiryaev, and G. L. Bychkov