A Layered Iron-Rich 2234-Type with a Mixed Valence of Iron: The Ferrimagnetic Tl-Doped Fe₂(Sr_2-εTl_ε)Sr₃Fe₄O_14.65

A new Tl-doped strontium ferrite Fe 2 (Sr 2-ϵ Tl ϵ )Sr 3 Fe 4 O 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 = α a⃗ * 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 [Fe 2 (Sr 2-ϵ Tl ϵ )O 3.4 ] ∞, built up of one double iron layer [Fe 2 O 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. Fe 2 (Sr 2-ϵ Tl ϵ )Sr 3 Fe 4 O 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.