Synthesis, structure and geometrically frustrated magnetism of the layered oxide-stannide compounds Fe(Fe3−xMnx)Si2Sn7O16

Abstract

Fe₄Si₂Sn₇O₁₆ has a unique crystal structure that contains alternating layers of Fe²⁺ ions octahedrally coordinated by O (oxide layer) and Sn (stannide layer), bridged by SiO₄ tetrahedra. The formula can be written as FeFe₃Si₂Sn₇O₁₆ to emphasise the distinction between the layers. Here, we report the changes in structure and properties as iron is selectively replaced by manganese in the oxide layer. Solid-state synthesis was used to produce polycrystalline samples of Fe(Fe_3−xMn_x)Si₂Sn₇O₁₆ for x ≤ 2.55, the structures of which were characterised using high-resolution synchrotron X-ray and neutron powder diffraction. Single-crystal samples were also grown at x = 0.35, 0.95, 2.60 and characterised by single crystal X-ray diffraction. We show that manganese is doped exclusively into the oxide layer, and that this layer contains exclusively magnetically active high-spin M²⁺ transition metal cations; while the stannide layer only accommodates non-magnetic low-spin Fe²⁺. All samples show clear evidence of geometrically frustrated magnetism, which we associate with the fact that the topology of the high-spin M²⁺ ions in the oxide layer describes a perfect kagomé lattice. Despite this frustration, the x = 0 and x = 2.55 samples undergo long-range antiferromagnetic ordering transitions at 3.0 K and 2.5 K, respectively.