Structural and Magnetic Characterization of perovskite type 112 compounds REBaCuCoO_5+d (RE = rare earths and ytrium).

Abstract:

    This thesis work describes the structure and magnetism of the REBaCuCoO_5+d compounds with RE = Y, La, Pr, Nd, Sm, Eu, Dy, Er and Tm as determined by synchrotron X rays and neutron powder diffraction and susceptibility measurements. Combined neutron and X-rays Rietveld refinement has been applied to obtain structural parameters that have shown to be extremely precise in some cases. The compounds with RE = Y, Dy, Er and Tm are isostructural and were refined in the typical 112 pervoskite type tetragonal space group P4/mmm. Both Cu and Co occupy different positions in the cell displaying Cu-O and Co-O bond distances consistent with elongated CuO₅ and almost symmetric CoO₅ pyramids. The magnetic moments of Cu(II) and LS-Co(III) atoms display antiferromagnetic ordering below the Néel temperature that was estimated to lie between 500 and 600K. The magnetic structure could be described by the bicolour I4’/m’m’m space group with a_M = a y c_M = 2c. This is consistent with magnetic moments aligned in the c-axis direction with an antiferromagnetic arrangement along xy plane and a +-|-+ arrangement along the z direction, which implies an antiferromagnetic coupling of atoms linked by the apical oxygen atom. The RE = Eu and Sm compounds, were described in the same structural arrangement but they display a bigger amount of nonstoichiometric oxygen, which tends to increase with the increasing size of the RE cation when going from Dy to Eu and from Eu to Sm. The RE = Pr and Nd compounds are also tetragonal with a P4/mmm structural arrangement. Stacking faults apparently affect the shapes and positions of the peaks, being impossible to adjust them with the algorithms already implemented to model this phenomenon. For this reason the structure are poorly determined. The presence of an important amount of nonstoichiometric oxygen (d ≈ 0.5) makes Cu(II) to oxidize to Cu(III) and this is associated with a change in the magnetic structure of these compounds being described in the bicolour I4’m’m space group with a_M = a y c_M = c. The LaBaCuCoO5_5+d compound displays a different structure, dominated by a pseudo-cubic arrangement of atoms which could be modelled in a orthorhombic Pmmm structure with 1´1´3a_P unit cell being a_P the typical perovskite cubic cell and the oxygen concentration d ≈ 0.5. The magnetic behaviour of this compound is similar to the one observed for Pr and Nd compounds. It is apparently possible to associate a decrease in the Néel temperature with the increase in the RE ionic radii.