A single crystal of GdBaCo 2 O 5.47 (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 a 1 . Critical exponent for the order parameter is found to be 0.33(1) = β .The superstructure reflections are about 2-4 orders of magnitude weaker than the basic ones. Their systematic extinction indicates the crystal symmetry change from to . 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 Co Pmmm Pmma 3+ ions. PACS numbers: 61.10.+Nz; 61.50.Ks; 61.66.Fn Since discovery of giant magnetoresistance in the oxygen-deficient layered perovskites RBaCo 2 O 5+x , where R is a rare earth [1], these materials attract high interest. Their orthorhombic structure at x ≈ 0.5 with the unit cell a 1 ≈ a p , a 2 ≈ 2a p , a 3 ≈ 2a p , where a p is parameter of the pseudocubic perovskite cell, is usually described by the Pmmm space group [2-5]. One can imagine the structure as a sequence of stacking plains [CoO 2 ][BaO][CoO 2 ][RO x ] along [0,0,1], which results in two types of the cobalt environment: CoO 5 pyramids and CoO 6 . octahedra. The nominal valence of cobalt at x = 0.5 is 3+. It is known [6] that the Co 3+ ion has a non-magnetic, or low-spin ground state (LS, ) as well as two excited states, the intermediate-spin (IS, ) and the high-spin (HS, ) ones. The energy differences are small enough to gain the excited states by the thermal fluctuations or due to the lattice change, which results in spin state transitions [7]. A metal-insulator (MI) transition has been also found, with the transition temperature for GdBaCo 0 6 2 g g e t 1 5 2 g g e t 2 4 2 g g e t 2 O 5+x 350 K < T MI < 370 K being dependent on the oxygen content of 0.4 < x < 0.47 [1, 2 8-10]. The transition is of the first order, which is indicated by a hysteresis of 8 K in resistivity for TbBaCo 2 O 5.4 [8].In spite of numerous studies of these materials, neither Co 3+ spin state nor nature of the MI transition is finally established. A spin-state transition coupled with the orbital degrees of freedom is suggested to be a driving force for the MI transition. The distribution of the IS e g orbitals (3x 2 -r 2 ) in pyramids and (3y 2 -r 2 ) in octahedral sites on cooling has been suggested as an origin of transition on the basis of structural studies of TbBaCo 2 O 5.5 [11]. On the other hand, it has been concluded that the transition to metallic phase in GdBaCo 2 O 5.5 is due to excitation of the LS-state electrons into e g band of the Co HS-state in octahedra; with Co in pyramids having IS both sides of T MI [9]. This conclusion has been made because of octahedron expansion of about 0.012(4) Å and simultaneous pyramid shrinking. The spin-state as well as the orbital ordering among one type of coordinating polyhedra was considered in a num...

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A giant softening by 30 cm −1 of the 490 and 620 cm −1 Jahn-Teller and breathing optical phonon modes is observed in Raman spectroscopy below the Curie temperature of single crystalline LaMnO 3+␦ ͑0.085ഛ ␦ ഛ 0.125͒. A pseudogaplike suppression of a continuum and a Fano antiresonance at 144 cm −1 appear below the charge-ordering temperature. Upon going through the antiferromagnetic/ferromagnetic insulating phase boundary a high-frequency maximum of three-peaks structure evolves to a unstructured, broadened maximum while undergoing a softening in the peak energy. This is interpreted in terms of the presence of fluctuating orbitals and mobile holes which form a stripelike state in the lightly doped, insulating manganites.

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