X-ray diffraction study of superstructure in GdBaCo2O5.5 -: art. no. 184105

A single crystal of GdBaCo2O5.47(2) has been studied by means of x-ray diffraction. Below the first-order metal-insulator transition, a continuous transition to a phase with a doubled orthorhombic unit cell is discovered at T-C = 341.5(7) by the appearance of very weak superstructure reflections. The critical exponent for the order parameter is found to be beta = 0.328(8), indicating an Ising pseudospin ordering in a 3D system. The systematic extinction of the superstructure reflections evidences the crystal symmetry change from Pmmm to Pmma, which explains the existence of a spontaneous moment. The integrated intensities are used to calculate the atomic displacements from their positions in the high-temperature phase. The cobalt-ligand distances in the ordered phase are discussed in terms of possible spin-state and/or orbital ordering of Co3+ ions.