Synthesis and Mössbauer characterization of Fe1+xCr 2-xO4 (0 ≤ x ≤ 2/3) spinel single crystals

Spinel single crystals belonging to the FeCr2O 4-Fe3O4 join were synthesized by a flux growth method (Na2B4O7 as flux, temperature range of 1200-900 °C and cooling rate of 4 °C/h). Crystals with compositions corresponding to Fe1+xCr2-xO4 (0 ≤ x ≤ 2/3) were obtained and successively investigated by a combined microchemical, diffractometric and spectroscopic approach. Fe-Cr-spinel single crystals produced are of different quality (in terms of size and shape). The flux content in the starting mixture showed to have the most relevant influence on the quality of synthetic products. Electron microprobe analysis evidenced a chemical heterogeneity for crystals of the same batch, with small crystals (down to 50 μm) being more homogeneous than the large ones. Mössbauer spectroscopy was used to determine the actual Fe3+/Fetot ratios and highlighted the absence of magnetic phases. The combined chemical and spectroscopic approach allowed to exclude any deviation from stoichiometry due to oxidation. In contrast with most of the existing literature, Mössbauer spectroscopy indicated the Fe2.5+ valence state to be present even for iron poor samples with composition close to the end member chromite.