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.