Mixed-valence hydrated iron fluoridotitanate: Synthesis, optics and calorimetry

Mixed-valence hydrated fluoridotitanate with idealized formula FeTiF6 center dot 6H(2)O is the main product of fluoride processing of natural mineral ilmenite with hydrofluoric acid. Its single crystals were grown and optical and calorimetric experiments were carried out in the temperature range 200-350 K. Charge re-distribution, Fe2+Ti4+ to Fe3+Ti3+, in the compound studied was evident in XPS spectra. A first order ferroelastic phase transition occurs at temperatures T-0 down arrow = 271.5 K, T-0 up arrow = 274 divided by 275.5 K with the symmetry change P (3) over bar <-> P (1) over bar. The structural transformation is accompanied by the appearance of a very weak optical anisotropy in the slice (001), a small anomaly of the heat capacity (Delta S = 1.5 J/mol K = 0.2R), and positive baric coefficients dT(0)/dp approximate to 30 + 10 K/GPa.