Structural and magnetic study ofPrBaCo2O5+δ(δ≃0.75)cobaltite

We have investigated the structural and magnetic properties of ${\mathrm{PrBaCo}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ $(\ensuremath{\delta}\ensuremath{\simeq}0.75)$ layered cobaltite. By means of neutron and synchrotron x-ray powder diffraction we have determined the ordered distribution of oxygen vacancies in ${\mathrm{PrO}}_{\ensuremath{\delta}}$ planes. This order doubles both $a$ and $b$ lattice parameters from the perovskite cell parameter, forming a tetragonal (rather than orthorhombic) structure. On cooling, a paramagnetic to ferromagnetic and an incomplete ferromagnetic to antiferromagnetic transition take place. This second transition leads to a coexistence of ferromagnetic and antiferromagnetic order, forming a canted $G$-type antiferromagnetic structure. Magnetic data above the ferromagnetic transition evidence a large value of the effective paramagnetic moment of $\mathrm{Co}$, implying that, well below room temperature, ${\mathrm{Co}}^{3+}$ ions in an octahedral environment present a high-spin state, in contrast to $R{\mathrm{BaCo}}_{2}{\mathrm{O}}_{5.5}$ cobaltites.