Synthesis and Magnetic Properties of a Ba₆Cu_24-x Fe _x Se₂₇ Solid Solution with the Bartonite Structure Type

A metal selenide resulting from a mixed-site Fe/Cu solid solution was synthesized, Ba₆Cu_24‑x Fe _x Se₂₇, in the composition range between Ba₆Cu₁₁Fe₁₃Se₂₇ (x = 13; 1) to Ba₆Cu₆Fe₁₈Se₂₇ (x = 18; 2). Its structure is derived from a parent mineral, i.e., naturally occurring bartonite K₆Fe_24‑x S₂₆(S,Cl), with the bartonite structure type in the I4/mmm space group and the general formula Ba₆ M ₂₄Se₂₇ (M = Fe, Cu). In contrast to known low-dimensional iron selenides consisting of FeSe₄ tetrahedra arranged in chains or layers, its structure is composed of a three-dimensional (3D) anionic framework of _∞ ³[M₂₄Se₂₆]^10-. This 3D framework is constructed from fluorite-type clusters that link via shared-vertices. Open channels are filled by charge-compensating Ba-(II) cations and isolated Se^2- anions in all crystallographic directions. At the compositional end limits of the solid solution synthesized here, the Fe cations possess an average oxidation state between +2.32 in 1 and +2 in 2. Magnetic measurements reveal that 2 exhibits ferrimagnetic order at least up to 350 K, with a secondary magnetic transition at ∼170 K. In summary, these results demonstrate the utilization of a mixed Cu/Fe solid solution to stabilize a 3D metal-selenide framework, hence opening the door to wider investigations of their intriguing magnetic properties.