Crystal structure and magnetic properties of lepidocrocite upon thermal transformation to hematite

Synthesized lepidocrocite (-FeOOH) has a unit cell of the rhombic system (a = 3.86 Å, b = 12.49, and c = 3.36). In the 230–250°C region the structure of lepidocrocite is reconstructed to cubic (a = 8.331 ± 0.003 A) maghemite (-Fe₂O₃), which at 375°C is transformed completely to hematite (-Fe₂O₃) with a hexagonal structure which becomes more perfect with further heating (a = 5.0330 ± 0.002 Å, c = 13.748 ± 0.002 Å). Lepidocrocite occurs in a paramagnetic state. The intermediate substance of its thermal transformation, maghemite, as a consequence of high dispersion, is characterized by superparamagnetic properties and the end product, hematite, owing to the presence of Dzialoshinski's moment, has weak ferromagnetism.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 84–90, May, 1972.