Iron(II)oxalate Dihydrate—Humboldtine: Synthesis, Spectroscopic and Structural Properties of a Versatile Precursor for High Pressure Research

Abstract

Iron(II)oxalate dihydrate FeC2O4 × 2 H2O—humboldtine is not only an important synthetic intermediate, but also a key building block for the preparation of various advanced materials. Interestingly, FeC2O4 × 2 H2O can be transformed readily into phase-pure siderite FeCO3. The importance of siderite for earth sciences, in particular for the understanding of the deep carbon cycle of our planet, is paramount. The availability of high-quality single crystals of FeC2O4 × 2 H2O is crucial for diffraction or spectroscopic studies at high pressure. The present article describes a versatile synthetic approach to single crystals of FeC2O4 × 2 H2O and its deuterated analogue starting from metallic iron together with a complete characterization of the products obtained. The same protocol has been employed successfully for the preparation of 57FeC2O4 × 2 H2O, as required for Möβbauer spectroscopy. In addition, the pressure-dependence of the crystal and molecular structure of the title compound was investigated up to p ≥ 20 GPa.