Crystal structure and magnetism of the double perovskites Sr3Fe2TeO9 and Ba3Fe2TeO9:: a neutron diffraction study

Sr3Fe2TeO9 and Ba3Fe2TeO9 double perovskites have been prepared in polycrystalline form by solid state reaction, in air. These materials have been studied by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure of Sr3Fe2TeO9 is tetragonal, space group I4/m, with a = b = 5.55902(4) angstrom, c = 7.885(1) angstrom, whereas Ba3Fe2TeO9 presents a hexagonal lattice, space group P6(3)/mmc, with a = 5.7670(1) angstrom, c = 14.1998(4) angstrom. The structure of Sr3Fe2TeO9, which can ideally be rewritten as Sr2Fe(Fe1/3Te2/3)O-6, can be described as the result of a single anti-phase tilting of the FeO6 and (Fe,Te)O-6 octahedra along the c axis, exhibiting a tilting angle of 4.5 degrees at RT. Ba3Fe2TeO9 shows a different stacking of the Fe/Te octahedra: the structure is constituted by dimer units of (Fe,Te)O-6 octahedra sharing a face along the c axis; the dimers are connected, sharing corners, by a single layer of FeO6 octahedra. In both Sr and Ba compounds a certain degree of antisite disordering has been detected, implying the presence of about 15% Te at Fe positions. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperatures of 717 and 711 K for the Sr and Ba compounds, respectively; however the magnetization isotherms at 2 K exhibit, for H = 50 kOe, maximum magnetization values close to 0.8 mu(B) f.u.(-1) and 0.35 mu(B) f.u.(-1) for Sr and Ba compounds respectively, although full saturation is not reached. The extremely weak magnetic scattering contribution observed on the low-temperature NPD patterns for Sr3Fe2TeO9 is in contrast with the well-established ferrimagnetic structures observed for other members of the Sr3Fe2B '' O-9 double perovskite series (B '' = U, Mo, W). This distinct behavior is discussed as a function of the chemical nature of the different B '' hexavalent cations.