Synthesis and properties of La18Fe5Cu4S26O8, a compound containing large magnetic clusters

Abstract

A new quinary oxysulfide, La₁₈Fe₅Cu₄S₂₆O₈, was synthesized by solid-state reaction, and is found to crystallize in a novel type structure. X-Ray diffraction on powder and single crystal samples reveal an orthorhombic lattice, which can be described with the Cmcm (no. 63) space group, with lattice constants a = 9.7999(2) Å, b = 26.1186(3) Å and c = 9.7767(1) Å. The matrix of the crystal structure is built up of layers of [La₉O₄]-units connected by sulfur ions and interspaced with transition metal arrangements. The structure exhibits a guest-in-host arrangement where the guest constitutes 1D chains of transition metal ions with substantial static disorder, which can be simulated using several split positions with partial occupancies. AC and DC magnetic susceptibility (χ) measurements reveal the formation of magnetic spin domains below ∼15 K; AC data indicate that the domain wall motions are thermally activated because Arrhenius-like frequency dependencies on both the real and imaginary parts of χ(T) are observed. However, the spin domains should have relatively large sizes because there is a significant entropy release during the domain formation, as indicated by specific heat measurements. Temperature-dependent ⁵⁷Fe-Mössbauer spectroscopy data reveal that the compound lacks magnetic order down to 20 K, but exhibits signatures of spin-freezing at 6 K. Hence, it is suggested that the title compound exhibits large magnetic clusters with significant spin dynamics at high temperatures, which freeze into a disordered state at low temperatures.