Spin reorientation and magnetic frustration in Fe32+δGe35−xSix with a kagome lattice broken by crystallographic intergrowth

Fe_32+δGe_35−xSi_x was synthesized using solid-state and chemical vapor transport reactions both in powder and single crystalline forms. Single crystal and high-resolution powder X-ray diffraction experiments revealed Fe_32+δGe_35−xSi_x to be a third member of the Fe_32+δGe_35−xE_x (E = p-element) family of ternary compounds alongside Fe_32+δGe₃₃As₂ and Fe_32+δGe_35−xP_x. Fe_32+δGe_35−xSi_x features a two-dimensional intergrowth structure of two parent structure types: MgFe₆Ge₆ and Co₂Al₅. Similarly to the other members, the stabilisation of the intergrowth structure in Fe_32+δGe_35−xSi_x occurs as a result of the p-element substitution in the MgFe₆Ge₆-type block. The intergrowth breaks the kagome net of MgFe₆Ge₆ into individual hexagrams, while providing additional layers of geometrically frustrated atomic arrangements. Magnetic measurements showed antiferromagnetic ordering at T_N ~ 150–160 K and spin reorientation below 80–90 K owing to the competition between magnetic interactions in the frustrated magnetic lattice of Fe_32+δGe_35−xSi_x.