Pseudo-Polymorphism in Layered FeS Intercalates: A Competition between Charged and Neutral Guest Species

Systematic synthesis studies of the formation of tetrahedral FeS-ethylenediamine intercalates resulted in the synthesis of a new compound, [Fe_9.4(2)S₁₀]­[Fe­(en)₃]_0.6(1)·en_0.9(3). The composition and complex crystal structure were determined based on a synergistic combination of elemental composition, decomposition behavior, high-resolution synchrotron X-ray diffraction and total scattering, ⁵⁷Fe Mössbauer spectroscopy, and electron diffraction. The structural model was derived based on a systematic comparison to the previously reported structures [Fe₈S₁₀]­[Fe­(en)₃]₁·en_0.5 and tetragonal FeS. The new compound has flat Fe_9.4S₁₀ layers, analogous to those in superconducting binary FeS. In the crystal structure of [Fe_9.4S₁₀]­[Fe­(en)₃]_0.6·en_0.9, the interlayer space is occupied by [Fe­(en)₃]²⁺ complexes and neutral ethylenediamine molecules in a ∼2:3 ratio. Interlayer species are not randomly oriented but ordered as evidenced by superstructural diffraction peaks in both high-resolution X-ray diffraction and electron diffraction patterns. Magnetic studies reveal no superconducting transition down to 2 K, indicating that the presence of minute amounts (∼6%) of iron vacancies at the Fe-S layer in [Fe_9.4S₁₀]­[Fe­(en)₃]_0.6·en_0.9 is still sufficient to shift the position of the Fermi level resulting in an adjustment of the properties. Our work shows the importance of detailed characterization of the crystal structure of intercalated compounds to understand the origin of the observed properties and develop proper structure–property relationships.