Synthesis and characterisation of the n=2 Ruddlesden–Popper phases Ln2Sr(Ba)Fe2O7 (Ln=La, Nd, Eu)

•Some Ruddlesden–Popper phases have been characterised.•Substitution on the A site influences cationic order.•The magnetic moment redirects with temperature A series of n=2 Ruddlesden–Popper phases A2B2O7 of composition Ln2Sr(Ba)Fe2O7 (Ln=La, Nd, Eu) have been prepared. La2SrFe2O7 and La2BaFe2O7 crystallise in the tetragonal space group I4/mmm. The structures of Eu2SrFe2O7 and Nd2SrFe2O7 are best described in space group P42/mnm. Substitution on the A site with smaller lanthanide- and larger alkaline metal- ions leads to enhanced cationic order in these phases and reflects increasing differences in cationic radii. All the compounds are antiferromagnetically ordered between 298 and 2K. In La2SrFe2O7 the magnetic moment lies along [110] at all temperatures between 298 and 2K whereas in La2BaFe2O7 the magnetic moment at 298K lies along the crystallographic x-axis but redirects from the [100] to the [110] direction between 210 and 190K and is retained in this direction until 2K. In Nd2SrFe2O7 the magnetic moment at 298K lies along [110] but rotates from [110] to [001] between 17 and 9K. A series of 57Fe Mössbauer spectra recorded from La2SrFe2O7 between 290 and 600K indicate a magnetic ordering temperature of TN≥535K.