Large single crystals of LnBaCo2O5.5: Initial nucleation, growth and study

Large and high-quality single crystals of LnBaCo2O5+x (Ln-112; Ln=Gd, Dy; 0 < x < 1) of rectangular shape have been grown from an overstoichiometric fluxed melt. We report how the approach to nucleation and flux growth under a variable temperature gradient and the use of MgO crucibles resulted in high-quality and large single crystals of these compounds of dimensions up to 20X12X1.0 mm3. We carried out high-resolution and resonance photoemission of the crystals. This allowed us to probe in great detail the highest energy bands, giving direct information on the mechanism of the metal-insulator transition and the character of the valence band states. By neutron and X-ray diffraction we have detected a structural transition in DyBaCo2O5.5 from the Pmmm to Pmma phase with heating. Four models of the spin structure which correspond to the Pmma space group symmetry and agree equally well with the experimental data are suggested for the high-temperature magnetically ordered phase in this compound.