Mössbauer Spectroscopic Investigation of FeII and FeIII 3,4,5‐Trihydroxybenzoates (Gallates) – Proposed Model Compounds for Iron‐Gall Inks

Iron gallates with iron in the oxidation states Fe 2+ and Fe 3+ were prepared and studied by Mössbauer spectroscopy, X‐ray diffraction, and IR spectroscopy. Fe III 3,4,5‐trihydroxybenzoate (gallate) Fe(C 7 O 5 H 4 ) · 2H 2 O, whose structure was first determined by Wunderlich , was obtained by the reaction of gallic acid and metallic iron or by oxidation of the Fe II gallate, which was obtained by the reaction of ferrous sulfate with 3,4,5‐trihydroxybezoic acid (gallic acid) under anoxic conditions. Trials to reproduce the hydrothermal preparation method of Feller and Cheetham show that the result depends crucially on the free gas volume in the reaction vessel. If there is no free volume one obtains the same Fe III gallate as in the other preparation methods. With a large free volume another compound was found to form whose composition and structure could not be determined. It could be specified only by Mössbauer spectroscopy. Fe III gallate, the Fe II gallate, and the new phase show magnetic ordering at liquid helium temperature.