Impact of structural features on pigment properties of α-Fe2O3 haematite

Various α-Fe2O3 haematite samples were synthesized by precipitation routes (under standard or hydrothermal conditions) followed by thermal treatments under air. The trigonal distortion (C3v point group) of the Fe3+ octahedral sites, which depends on the synthesis route and thermal treatment, was investigated by X-ray diffraction, Moessbauer spectroscopy and visible-near infrared (Vis-NIR) spectroscopy. The correlation between diffuse reflectance spectra and structural features of the haematite samples is reported and discussed herein. The slight increase of the average distortion of the Fe3+ octahedral sites, which depends on the annealing temperature of the precipitated sample, directly linked to the crystallite size, contrasts with the larger reduction of the sites distortion for the compound prepared by hydrothermal route due to the occurrence of hydroxyl groups substituted for O2- anions as well as Fe3+ cationic vacancies. On a local point of view, as shown by Moessbauer spectroscopy, the Fe3+ octahedral sites distortion decreases from the centre towards the surface of the grains. Then the smaller the grain size, the lower the average site distortion. Finally, the reduction of the octahedral distortion was directly correlated to the two Fe-O charge transfer bands in the visible range and the colour of as-prepared haematites. - Graphical abstract: The reddish colour of Fe2O3 haematite was analysed in regard to its structural features, especially in regard to the Fe3+ octahedral site distortion, which is progressively less and less distorted from the centre towards the surface of the grains