Magnetic Structures of Orthorhombic Li₂M(SO₄)₂ (M = Co, Fe) and Li_<i>x</i>Fe(SO₄)₂ (<i>x</i> = 1, 1.5) Phases

We report herein on the magnetic properties and structures of orthorhombic Li2M­(SO4)2 (M = Co, Fe) and their oxidized phases LixFe­(SO4)2 (x = 1, 1.5), which were previously studied as potential cathode materials for Li-ion batteries. The particular structure of these orthorhombic compounds (space group Pbca) consists of a three-dimensional network of isolated MO6 octahedra enabling solely super-super-exchange interactions between transition metals. We studied the magnetic properties of these phases via temperature-dependent susceptibility measurements and applied neutron powder diffraction experiments to solve their magnetic structures. All compounds present an antiferromagnetic long-range ordering of the magnetic spins below their Néel temperature. Their magnetic structures are collinear and follow a spin sequence (+ + – – – – + +), with the time reversal associated with the inversion center, a characteristic necessary for a linear magneto-electric effect. We found that the orientation of the magnetic moments varies with the nature of M. While Li2Co­(SO4)2 and Li1Fe­(SO4)2 adopt the magnetic space group Pb′c′a′, the magnetic space group for Li2Fe­(SO4)2 and Li1.5Fe­(SO4)2 is P1121′/a, which might hint for a possible monoclinic distortion of their nuclear structure. Moreover we compared the orthorhombic phases to their monoclinic counterparts as well as to the isostructural orthorhombic Li2Ni­(SO4)2 compound. Finally, we show that this possible magneto-electric feature is driven by the topology of the magnetic interactions.本研究报道了正交晶系（orthorhombic）Li₂M(SO₄)₂（M=Co、Fe）及其氧化相LiₓFe(SO₄)₂（x=1、1.5）的磁学性质与晶体结构，这类化合物此前曾被作为锂离子电池（Li-ion batteries）的潜在正极材料开展研究。该类正交晶系化合物的空间群（space group）为Pbca，其独特结构由孤立的MO₆八面体（MO₆ octahedra）构成三维网络，仅允许过渡金属间发生超超交换相互作用（super-super-exchange interactions）。本研究通过温度依赖磁化率测试（temperature-dependent susceptibility measurements）表征了上述物相的磁学性质，并借助中子粉末衍射（neutron powder diffraction）实验解析了其磁结构。所有化合物在其奈尔温度（Néel temperature）以下均表现出磁自旋的反铁磁长程有序（antiferromagnetic long-range ordering）。其磁结构为共线型，自旋排布遵循(+ + – – – – + +)的序列，且时间反演（time reversal）与反演中心（inversion center）相关联，这一特征是实现线性磁电效应（magneto-electric effect）的必要条件。研究发现磁矩的取向随M的种类发生变化：Li₂Co(SO₄)₂与Li₁Fe(SO₄)₂的磁空间群（magnetic space group）为Pb′c′a′，而Li₂Fe(SO₄)₂与Li₁.₅Fe(SO₄)₂的磁空间群则为P1121′/a，这一结果暗示其核结构（nuclear structure）可能存在单斜畸变（monoclinic distortion）。此外，本研究将上述正交晶系物相与其单斜对应物相，以及同构的（isostructural）正交晶系Li₂Ni(SO₄)₂化合物进行了对比。最后，本研究证实该潜在磁电效应特征由磁相互作用的拓扑结构所主导。