A Layered Iron-Rich 2234-Type with a Mixed Valence of Iron: The Ferrimagnetic Tl-Doped Fe2(Sr2-εTlε)Sr3Fe4O14.65

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• 作者：Christophe Lepoittevin ; Sylvie Malo ; Ninh Nguyen ; Sylvie Hebert ; Gustaaf Van Tendeloo ; Maryvonne Hervieu
• 刊名：Chemistry of Materials
• 出版年：2008
• 出版时间：October 28, 2008
• 年：2008
• 卷：20
• 期：20
• 页码：6468-6476
• 全文大小：755K
• 年卷期：v.20,no.20(October 28, 2008)
• ISSN：1520-5002

文摘

A new Tl-doped strontium ferrite Fe₂(Sr_2-ϵTl_ϵ)Sr₃Fe₄O_14.65, with an original structure, has been synthesized and structurally characterized by powder X-ray diffraction and transmission electron microscopy. The TGA and Mössbauer studies evidence a mixed valence of iron. The structure exhibits a commensurate modulation, with a F-type subcell a ≈ b ≈ 5.4 Å (≈ a_p√2), c ≈ 42 Å with a modulation vector q⃗ = αa⃗* with α = 0.4. The supercell parameters have been refined as a= 27.1101(8) Å, b= 5.5187(2) Å and c= 42.0513(9) Å, in the space group Fmmm. The electron diffraction and electron microscopy data of this novel ferrite show that it can be described as a Fe_Tl-2234-type structure corresponding to the intergrowth of a quadruple perovskite slice [(SrFeO_2.8)₄], with a complex rock salt related slice [Fe₂(Sr_2-ϵTl_ϵ)O_3.4]_∞, built up of one double iron layer [Fe₂O_2.4] sandwiched between two [SrO] layers. The HRTEM images show that the oxygen atoms and vacancies are randomly distributed in the perovskite layers while the HAADF STEM images evidence the absence of Tl segregation in the matrix. Fe₂(Sr_2-ϵTl_ϵ)Sr₃Fe₄O_14.65 exhibits a very large value of χ (11emu/mol) at 5 K, which remains large at 400 K; the M(H) loop presents a shape characteristic of ferrimagnetism, with a large coercive field of 0.3 T. The value of magnetization saturates at 400 K at 0.68 μ_B/Fe. At 10 K, the value of magnetization reaches a maximum of 2 μ_B/Fe. The resistivity presents a semiconducting-like behavior, with ρ ∼ 800 Ω·cm at 300 K.