Hydrothermal synthesis of nanodispersed "iron mica" from yellow iron oxide pigment

The temperatures within the 150-200 °C interval of preliminary heat treatment of yellow iron oxide pigment (the a-FeOOH phase) influence the disperse composition of “iron mica” (the a-Fe2O3 phase with platelet crystals), which is formed at hydrothermal treatment (230±5 °C) of the samples of yellow iron oxide pigment in aqueous solution of potassium hydroxide with 3.5 M concentration. It has been found that increasing temperature of preliminary heat treatment from 150 to 200 °C leads to decreasing sizes of coherent scattering regions, calculated from the X-ray phase analysis data, as well as the mean size of the d crystals of “iron mica” (in the plate plane), observed with the use of scanning electron microscope, by ~ 2 and ~ 20 times, respectively, compared to the samples without preliminary heat treatment. Significant change in the form of the crystals of initial (a-FeOOH has needle-like crystals) and resultant phases (a-Fe2O3 has platelet crystals) during phase transition testifies that the transformation of a-FeOOH into a-Fe2O3 proceeds according to the “dissolution - precipitation” mechanism, by way of dissolution of the phase that is nonequilibrial at the given thermodynamic conditions (a-FeOOH), formation of nuclei and growth of the equilibrium phase crystals (a-Fe2O3). The conclusion has been made concerning formation of the a-Fe2O3 nuclei on the surface of the yellow iron oxide crystals in the process of preliminary heat treatment. The number of the formed nuclei is greater at higher temperature t and duration t of the preliminary heat treatment. These parameters (t and t) let intentionally vary the number of the α-Fe2O3 nuclei, and, consequently, the disperse composition of “iron mica”.