X-ray diffraction study of superstructure in GdBaCo{sub 2}O{sub 5.5}

A single crystal of $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5.47(2)}$ 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 $\ensuremath{\beta}=0.328(8)$, 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 ${\mathrm{Co}}^{3+}$ ions.