Temperature-dependent crystal structure refinement and ⁵⁷Fe Mössbauer spectroscopy of Cu₂Fe₂Ge₄O₁₃

The germanate compound Cu₂Fe₂Ge₄O₁₃, dicopper diiron germanate, was synthesized by solid-state reaction at 1403 K and ambient pressure. There is no change of space-group symmetry between 10 and 900 K. Between 40 K and room temperature the a lattice parameter shows a negative thermal expansion which can be connected to a decreasing Cu—Cu interatomic distance. Above room temperature all the lattice parameters are positively correlated with temperature. Among the structural parameters several alterations with temperature occur, which are most prominent for the distorted Fe³⁺ octahedral site. Besides an increase of the average bond length and of the interatomic Fe—Fe distances, distortional parameters also increase with temperature, while the average Cu—O bond length remains almost constant between 100 and 900 K, as do the average Ge—O distances. ⁵⁷Fe Mössbauer spectroscopy was used to detect long-range magnetic ordering in Cu₂Fe₂Ge₄O₁₃. While around 100 K, which is the temperature at which a broad maximum is observed in the magnetic susceptibility, no magnetic ordering was detected in the Mössbauer spectrum, below 40 K a narrow split sextet is developed which is indicative of a three-dimensional magnetic ordering of the sample.