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

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Abstract

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, Mössbauer 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 Mössbauer 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 Fesingle bondO 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.

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Introduction

α-Fe2O3 is an inorganic red pigment largely used for several industrial applications, for instance colouring paints, plastics and enamels, thanks to its low price, low toxicity, and high thermal and chemical stability [1], [2]. Nevertheless, the dark-red colour of this mineral can strongly depend on precursors or synthesis routes [3]. In iron (III)-rich oxides such as haematite or spinels, the intense reddish-brown colour is due to an almost total absorption of the high-energy region of the visible spectrum [400–550 nm] and due to an important reflectivity in the low-energy part of the visible spectrum [550–800 nm] [4], [5], [6]. In a previous paper [7], visible–near infrared (Vis-NIR) absorption properties of haematite and spinel ferrites (AFe2O4) were correlated to their structural parameters. Even though numerous authors consider that all absorption bands in ferrites spinel or haematite result from Fe3+ 3d crystal field (CF) transitions [5], [6], [8], [9], in our recent study the two main absorption edges (band-gaps) occurring in the visible range [400–800 nm] were attributed to ligand to metal 2p(O2−)→3d(Fe3+) charge transfers. C3v trigonal distortion of [FeO6] octahedra in haematite leading to an additional d orbitals splitting is at the origin of this double charge transfer. Then, it seemed obvious that the octahedral sites distortion is directly linked to the energy positions of the two band-gaps as well as the two d–d intra-atomic transitions in Vis–NIR range. The aim of this work is to prepare various α-Fe2O3 compounds synthesized via different routes and/or with different thermal treatments and to characterize the octahedral site distortion of the various haematite compounds.

The crystal structures of these haematite samples have been studied by powder X-ay diffraction (Rietveld refinement). The local environments of the Fe3+ cations and the magnetic behaviour have been investigated by Mössbauer spectroscopy. Finally, the correlation of their Vis–NIR absorption spectra with their structural features will be presented.

Section snippets

Preparation

α-Fe2O3 compounds were prepared by a precipitation process in basic medium (i) or by hydrothermal route assisted by microwave (ii).

  • (i)

    A 7.2 M NH4OH solution was added to a 0.5 M aqueous solution of iron nitrate (Fe(NO3)3·9H2O; Aldrich) in order to precipitate metal ions with hydroxide form Fe(OH)3. According to the iron Pourbaix diagram, iron hydroxide is stable in a large pH range, from 4.5 to 10. The working pH is 9.5. The brown precipitate was dried overnight at 100 °C. Then, in order to obtain

Structural description

The haematite phase α-Fe2O3 crystallizes in hexagonal symmetry with R-3c space group related to the corundum-type structure. The Fe3+ iron cations are distributed with an ordering of 2/3 of the octahedral sites (12c wickoff positions) within the framework of a hexagonal close-packed array of O2− ions. The crystallographic network can be described as “chains” of octahedral sites directed along the c-axis and constituted by [Fe2O9] dimers—two face-sharing Fe3+ octahedral sites—separated from each

Conclusion

Depending on the synthesis conditions, the composition and/or crystallite size of haematites were controlled, allowing to tune the trigonal distortion of the Fe3+ octahedral sites and so the colour of the obtained samples. Indeed, the evolution of the trigonal distortion directly influences the crystal field intensity and splitting, and so the energy positions of the charge transfer bands as well as the d–d absorption bands located in the visible–NIR range. It is an outstanding example where

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