Université Lille Nord de France, UCCS, UMR-CNRS 8181, ENSCL-USTL, Villeneuve d'Ascq, France.
J Am Chem Soc. 2013 Sep 4;135(35):13023-9. doi: 10.1021/ja404697b. Epub 2013 Aug 22.
BaFe2(PO4)2 was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO6 octahedra containing high-spin Fe(2+) ions (S = 2) are isolated by PO4 groups and Ba(2+) cations. BaFe2(PO4)2 has a trigonal R-3 structure at room temperature but adopts a triclinic P-1 structure below 140 K due to the Jahn-Teller (JT) instability arising from the (t2g)(4)(eg)(2) configuration. The triclinic crystal structure was refined to find significantly distorted Fe(2+)O6 octahedra in the honeycomb layers while the distortion amplitude QJT was estimated to 0.019 Å. The JT stabilization energy is estimated to be ∼7 meV per formula unit by DFT calculations. Below ∼70 K, very close to the ferromagnetic transition temperature Tc = 65.5 K, the structure of BaFe2(PO4)2 returns to a trigonal R-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe(2+), as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above Tc , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe2(PO4)2 is uniaxial (g⊥ = 0).
BaFe2(PO4)2 最近通过水热合成法制备，并被确定为首个二维（2D）伊辛铁磁氧化物，其中由 PO4 基团和 Ba(2+)阳离子隔离的由共享边的 FeO6 八面体组成的蜂窝状层含有高自旋 Fe(2+)离子（S = 2）。BaFe2(PO4)2 在室温下具有三方 R-3 结构，但由于（t2g）（4）（eg）（2）构型引起的 Jahn-Teller（JT）不稳定性，在低于 140 K 时采用三方 P-1 结构。对三方晶系结构进行了精修，发现蜂窝状层中 Fe(2+)O6 八面体明显变形，而 JT 变形幅度 QJT 估计为 0.019 Å。通过 DFT 计算估计 JT 稳定能约为每个分子式单位 7 meV。在约 70 K 以下，非常接近铁磁转变温度 Tc = 65.5 K，在存在显著铁磁畴的情况下，BaFe2(PO4)2 的结构返回到三方 R-3 结构。这种罕见的重入结构转变伴随着 Fe(2+)的四极分裂的不连续变化，这是由穆斯堡尔光谱确定的。EPR 测量表明，在高于 Tc 的温度下存在磁畴，这是铁磁 2D Ising 系统的预期结果，并支持 BaFe2(PO4)2 的磁性是单轴的（g⊥= 0）。
