Low temperature fluorination of Sr3Fe2O7-x with polyvinylidine fluoride: An X-ray powder diffraction and Mossbauer spectroscopy study

Fluorination of the Ruddlesden Popper phase, Sr3Fe2O7-x by heat treatment with polyvinylidine fluoride (PVDF) gives a range of novel oxide fluoride compounds. Fluorination with 1 mole equivalent PVDF leads to a filling of the normal Ruddlesden Popper structure anion sites and a material of composition Sr3Fe2O5+xF2-x (x≈0.28(4)) which contains both Fe4+ and Fe3+. Increasing the amount of PVDF to 2 mole equivalent leads to an increase in anion content due to filling of half the interstitial sites within the structure, with iron being completely reduced to Fe3+ leading to a composition Sr3Fe2O4F4. An increase in the amount of PVDF to ≈3 mole equivalent leads to a further increase in unit cell volume, attributed to complete filling of the interstitial sites and a composition Sr3Fe2O3F6. 57Fe Mössbauer spectra in the temperature range 10 to 300K demonstrated the complexity of the magnetic interactions in each of the three phases which reflect different local compositions of oxygen and fluorine around the iron ions thus influencing the superexchange pathways.