Hudspeth J M, Goossens D J, Studer A J, Withers R L, Norén L
Department of Physics, The Australian National University, Canberra, ACT 0200, Australia.
J Phys Condens Matter. 2009 Mar 25;21(12):124206. doi: 10.1088/0953-8984/21/12/124206. Epub 2009 Feb 25.
The crystal and magnetic structures of LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa(2)Fe(3)O(8) and NdCa(2)Fe(3)O(8) were made up of stacked perovskite-like layers of FeO(6) octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca(2)Fe(2)O(5)) structure type. We have gone further and determined a likely space group for this Grenier phase and determined the magnetic structure of the compounds at room temperature. The space group is found to be P 2(1)ma (b axis as the long axis), and the crystal structure has been refined, subject to the stacking faulting along the long axis that is apparent in electron diffraction patterns. The magnetic structure of LaCa(2)Fe(3)O(8) is shown to consist of antiferromagnetically ordered Fe(3+) ions on a collinear G-type antiferromagnetic structure, with the magnetic moments most likely (anti)parallel with the c axis, and of magnitude 3.4 ± 0.2μ(B) (3.6 ± 0.2μ(B) for NdCa(2)Fe(3)O(8)). The result is reasonable given the magnetic structures of the end members of the La(1-x)Ca(x)FeO(3) series, LaFeO(3) (x = 0) and Ca(2)Fe(2)O(5) (x = 1).
利用X射线、中子和电子衍射相结合的方法，确定了LaCa₂Fe₃O₈和NdCa₂Fe₃O₈的晶体结构和磁结构。人们早就认为，LaCa₂Fe₃O₈和NdCa₂Fe₃O₈是由FeO₆八面体的钙钛矿状层堆叠而成，每隔三层就被一层四面体配位的Fe层取代，有点类似于钙铁石（Ca₂Fe₂O₅）结构类型的变体。我们进一步确定了这种格雷尼尔相可能的空间群，并确定了化合物在室温下的磁结构。发现空间群为P 2(1)ma（以b轴为长轴），并且根据电子衍射图中明显的沿长轴的堆垛层错对晶体结构进行了精修。结果表明，LaCa₂Fe₃O₈的磁结构由共线G型反铁磁结构上反铁磁有序的Fe³⁺离子组成，磁矩很可能（反）平行于c轴，大小为3.4±0.2μ(B)（NdCa₂Fe₃O₈为3.6±0.2μ(B)）。考虑到La(1 - x)Ca(x)FeO₃系列的端成员LaFeO₃（x = 0）和Ca₂Fe₂O₅（x = 1）的磁结构，该结果是合理的。
