Structural characterisation of REBaCo2O6−δ phases (RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho)

REBaCo2O6−δ (RE = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) phases were prepared by solid-state reaction at 1100 °C. Post-reaction heat treatments were carried out to alter the oxygen contents, which were determined by H2-reduction thermogravimetry. Three closely-related structure types were observed, with the general sequence: tetragonal (δ⩾0.6) to orthorhombic (0.6⩾δ⩾0.4) to tetragonal (0.4⩾δ) with increasing oxygen content. Rietveld refinement of combined X-ray and neutron powder diffraction data of NdBaCo2O6−δ was carried out for four values of δ. Oxygen deficiency is accommodated by partial occupancy of two sets of oxygen sites, giving a range of coordination environments for Co. Electron diffraction patterns revealed a superlattice ∼2ao×4ao×co with Cmmm symmetry, where ao and co refer to the (pseudo) tetragonal subcell. Tweed-like ferroelastic domains were routinely observed, suggesting that compositions that appear macroscopically tetragonal by X-ray diffraction are in fact orthorhombic with a≈b.