Impact of Mn³⁺ upon Structure and Magnetism of the Perovskite Derivative Pb_2–<i>x</i>Ba_<i>x</i>FeMnO₅ (<i>x</i> ∼ 0.7)

On the basis of the Mn3+ for Fe3+ substitution in Pb2–xBaxFe2O5, a novel oxide Pb1.3Ba0.7MnFeO5 has been synthesized at normal pressure. Though it belongs to the same structural family, the mixed “MnFe” oxide exhibits a very different structural distortion of its framework compared to the pure “Fe2” oxide, due to the Jahn–Teller effect of Mn3+. Combined neutron diffraction, high resolution electron microscopy/high angle annular dark field–scanning transmission electron microscopy (HAADF–STEM) investigations allow the origin of this difference to be determined. Here we show that the MO6 octahedra of the double perovskite layers in the “MnFe” structure exhibit a strong tetragonal pyramidal distortion “5 + 1”, whereas the “Fe2” structure shows a tetrahedral distortion “4 + 2” of the FeO6 octahedra. Similarly, the MO5 polyhedra of the “MnFe” structure tend toward a tetragonal pyramid, whereas the FeO5 polyhedra of the “Fe2” structure are closer to a trigonal bipyramid. Differently from the oxide Pb2–xBaxFe2O5, which is antiferromagnetic, the oxide Pb1.3Ba0.7MnFeO5 exhibits a spin glass behavior with Tg ∼ 50 K in agreement with the disordered distribution of the Mn3+ and Fe3+ species.基于在Pb₂₋ₓBaₓFe₂O₅中以Mn³⁺取代Fe³⁺的思路，我们在常压下合成了一种新型氧化物Pb₁.₃Ba₀.₇MnFeO₅。尽管该氧化物与纯‘Fe₂’氧化物同属一个结构家族，但由于Mn³⁺的姜-泰勒效应（Jahn–Teller effect），这种混合‘MnFe’氧化物的骨架结构畸变与纯‘Fe₂’氧化物存在显著差异。通过联用中子衍射、高分辨电子显微镜与高角环形暗场扫描透射电子显微镜（HAADF–STEM）表征手段，我们明确了该结构差异的起源。本研究表明，‘MnFe’结构中双钙钛矿（double perovskite）层的MO₆八面体呈现出强烈的‘5+1’型四方锥畸变，而‘Fe₂’结构中的FeO₆八面体则表现为‘4+2’型四面体畸变。同理，‘MnFe’结构中的MO₅多面体趋近于四方锥构型，而‘Fe₂’结构中的FeO₅多面体则更接近三角双锥构型。与具有反铁磁性的Pb₂₋ₓBaₓFe₂O₅不同，Pb₁.₃Ba₀.₇MnFeO₅展现出自旋玻璃行为，其自旋玻璃转变温度Tg约为50 K，这与Mn³⁺与Fe³⁺离子的无序分布结果一致。