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Determination des structures de deux ferrites mixtes nouveaux de formule BaLa2Fe2O7 et SrTb2Fe2O7
Authors:D Samaras  A Collomb  JC Joubert
Institution:Laboratoire des Rayons-X, C.N.R.S., B.P. N° 166—Centre de Tri, 38042 Grenoble Cédex, France
Abstract:The structures of the two new ferrites BaLa2Fe2O7 and SrTb2Fe2O7 with tetragonal symmetry have been resolved by X-ray and neutron diffraction performed on powder samples. In both compounds the arrangement of atoms present a close resemblance with the idealized Sr3Ti2O7 structure. It consists of a packing along the c axis of two different blocks. One is formed by the adjunction of two perovskite cells and the other one by a halved rocksalt Sr(Ba)O cell. The iron cation lattice is built by infinite double layers perpendicular to the c axis with the shortest Fe3+Fe3+ distances inside the double layers much shorter than between the layers. In BaLa2Fe2O7, Ba2+ is located on a regular 12-coordinated site and La3+ in a regular 9-coordinated polyhedron. Fe3+ is surrounded by five oxygen neighbours at 1.98 Å, building a rather regular tetragonal pyramid, with a sixth oxygen at 2.25 Å. In SrTb2Fe2O7, Sr has only eight close neighbours at ~2.80 Å and four more distant at 3.15 Å. Tb3+ has seven close neighbours, six building a distorted octahedron with the largest triangular face capped by the seventh oxygen. Fe3+ again has five neighbours, but due to the lowering of the symmetry, the square pyramid has become a distorted trigonal bipyramid.
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