Anion manipulation in perovskite-related materials

Sr₂Co₂O₅ with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters \(a\) = 5.4639(3) Å, \(b\) = 15.6486(8) Å and \(c\) = 5.5667(3) Å based on refinement of NPD data collected at 4K. Electron microscopy revealed L R L R intralayer ordering of chain orientations which requires a doubling of the unit cell along the \(c\) parameter, consistent with the assignment of the space group \(Pcmb\). However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group \(Imma\) appears more appropriate. Low-temperature fluorination of this material and the related brownmillerite phases Sr₂CoFeO₅ and Sr₂Fe₂O₅ has been found to give different products depending on the starting material. Sr₂Co₂O₅ and Sr₂CoFeO₅ are amenable to fluorine insertion reactions with corresponding oxidation of transition metal cations, forming cubic perovskite phases of the nominal compositions SrCoO_2.5F_0.5 (\(a\) = 3.8574(3) Å) and SrCo_0.5Fe_0.5O_2.5F_0.5 (\(a\) = 3.872(2) Å). Fluorination of Sr₂Fe₂O₅, however, results in a 1:1 mixture of two phases that were assigned to SrFeO₂F (\(a\) = 3.9481(3) Å) and SrFeO_3-δ (\(a\) = 3.8775(3) Å). Fluorine insertion into the oxygen defect superstructure manganite Sr₂MnO_3.5+x has been shown by 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 giving a K₂NiF₄ type structure (\(a\) = 3.8210(1) Å and \(c\) = 12.686(1) Å). Ce₂MnN₃F_2-δ with tetragonal symmetry (\(P4/nmm\) \(a\) = 3.8554(4) Å and \(c\) = 13.088(4) Å based on neutron powder diffraction) and a structure related to LaSrMnO₄F has been synthesised via low-temperature fluorination of the ternary nitride Ce₂MnN₃. Two F^- anions are inserted but no F^-/N3^- substitution takes place, instead a structural rearrangement whereby one F^- expands the manganese coordination from four to six occurs and the second F^- inserts in alternate layers of interstitial sites, in a staged fashion. The effective magnetic moment in Ce₂MnN₃F_2-δ, μ_eff = 5.38 μ_B, is consistent with an intermediate value between that of Mn³⁺ (4.9 μ_B) and Mn²⁺ (5.9 μ_B) supporting the proposed stoichiometry, Ce₂MnN₃F_2-δ.

Name dc:type.qualificationname

d_ph

Level dc:type.qualificationlevel

d_ph

Grantor dc:publisher.institution

University of Birmingham

Year dc:date.issued

2009

Author dc:creator

• Sullivan, Eirin Courtney

• QD Chemistry