Fluorine intercalation in the n = 1 and n = 2 layered manganites Sr 2MnO3.5+x and Sr3Mn2O6

Fluorine insertion into the oxygen defect superstructure manganite Sr ₂MnO_3.5+x has been shown by transmission electron microscopy (TEM) to result in two levels of fluorination. In the higher fluorine content sections, the fluorine anions displace oxygen anions from their apical positions into the equatorial vacancies, thus destroying the superstructure and reverting to a K₂NiF₄-type structure (a = 3.8210(1) a and c = 12.686(1) a). Conversely, lower fluorine content sections retain the Sr ₂MnO_3.5+x defect superstructure, crystallising in the P2₁/c space group. Fluorine intercalation into the reduced double-layer manganite Sr₃Mn₂O₆ occurs in a step-wise fashion according to the general formula Sr₃Mn ₂O₆F_y with y = 1, 2, and 3. It is proposed that the y = 1 phase (a = 3.815(1) a, c = 20.29(2) a) is produced by the filling of all the equatorial oxygen vacancies by fluorine atoms whilst the y = 2 phase (a = 3.8222(2) a, c = 21.2435(3) a) has a random distribution of fluorine anions throughout both interstitial rocksalt and equatorial sites. Neutron powder diffraction data suggest that the fully fluorinated y = 3 phase (a = 3.8157(6) a, c = 23.666(4) a) corresponds to the complete occupation of all the equatorial oxygen vacancies and the interstitial sites by intercalated fluorine.