title: Fluorine insertion into the Ruddlesden-Popper phase La2BaFe2O7: the structure and magnetic properties of La2BaFe2O5F4.
abstract: Fluorination of the n = 2 Ruddlesden-Popper phase La2BaFe2O7 occurs at ~300 °C in flowing 10% F2 in N2 to form La2BaFe2O5F4. This oxide fluoride contains 2F(-) ions in interstitial sites within the rocksalt regions and 2F(-) ions that have substituted for O(2-) ions in apical sites within the rocksalt layers. The fluorination results in an expansion along c of 7.6% to yield a tetragonal unit cell of dimensions a = 3.96237(7) Å, c = 22.3972(5) Å. The structure and magnetic properties have been examined by Mössbauer spectroscopy, neutron powder diffraction and magnetic susceptibility measurements. La2BaFe2O5F4 becomes antiferromagnetically ordered at temperatures below ~500 K, and the magnetic order shows a striking resemblance to that observed in La2BaFe2O7. The magnetic moments on Fe(3+) are perpendicular to [001] and aligned along ±{100} directions above 300 K, but at temperatures below 200 K, they rotate by 45° to lie along ±{110}. Mössbauer spectroscopy suggests the presence of Fe(3+) within the primary phase, but also indicates that fluorination results in some particle fragmentation to form a paramagnetic component of the fluorinated material.
authors.affiliation: School of Chemistry, The University of Birmingham, Birmingham B15 2TT, UK. c.greaves@bham.ac.uk.
authors.forename: Nicola N M
authors.initials: NN
authors.lastname: Gurusinghe
authors.forename: Julia C
authors.initials: JC
authors.lastname: Fones
authors.forename: Jose F
authors.initials: JF
authors.lastname: Marco
authors.forename: Frank J
authors.initials: FJ
authors.lastname: Berry
authors.forename: Colin
authors.initials: C
authors.lastname: Greaves