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

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 2 K. In La2SrFe2O7 the magnetic moment lies along [1 1 0] at all temperatures between 298 and 2 K whereas in La2BaFe2O7 the magnetic moment at 298 K lies along the crystallographic x-axis but redirects from the [1 0 0] to the [1 1 0] direction between 210 and 190 K and is retained in this direction until 2 K. In Nd2SrFe2O7 the magnetic moment at 298 K lies along [1 1 0] but rotates from [1 1 0] to [0 0 1] between 17 and 9 K. A series of 57Fe Mössbauer spectra recorded from La2SrFe2O7 between 290 and 600 K indicate a magnetic ordering temperature of TN ≥ 535 K.