Fluorine insertion into the Ruddlesden-Popper phase La2BaFe2O7: the structure and magnetic properties of La2BaFe2O5F4

Fluorination of the n = 2 Ruddlesden-Popper phase La 2 BaFe 2 O 7 occurs at ∼300 °C in flowing 10% F 2 in N 2 to form La 2 BaFe 2 O 5 F 4 . 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. La 2 BaFe 2 O 5 F 4 becomes antiferromagnetically ordered at temperatures below ∼500 K, and the magnetic order shows a striking resemblance to that observed in La 2 BaFe 2 O 7 . 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. Fluorination of La 2 BaFe 2 O 7 occurs to form La 2 BaFe 2 O 5 F 4 containing both interstitial and substitutional fluoride ions. The Fe 3+ ions are not oxidised.