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ARTICLE IN PRESS Journal of Solid State Chemistry 181 (2008) 2697– 2704 Contents lists available at ScienceDirect Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc Impact of structural features on pigment properties of a-Fe2O3 haematite ´ N. Pailhe, A. Wattiaux, M. Gaudon Ã, A. Demourgues ` ´ ´ Institut de Chimie de la Matiere Condensee de Bordeaux, UPR 9048 CNRS - Universite de Bordeaux 1, 87 Avenue du Docteur Schweitzer, Pessac Cedex 33608, France a r t i c l
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  Impact of structural features on pigment properties of  a -Fe 2 O 3 haematite N. Pailhe´, A. Wattiaux, M. Gaudon à , A. Demourgues Institut de Chimie de la Matie`re Condense´e de Bordeaux, UPR 9048 CNRS - Universite´de Bordeaux 1, 87 Avenue du Docteur Schweitzer, Pessac Cedex 33608, France a r t i c l e i n f o  Article history: Received 4 March 2008Received in revised form17 June 2008Accepted 19 June 2008Available online 1 July 2008 Keywords: Red pigmentsHaematiteX-ray diffractionMo¨ssbauer spectroscopyVis–NIR spectroscopy a b s t r a c t Various a -Fe 2 O 3 haematite samples were synthesized by precipitation routes (under standard orhydrothermal conditions) followed by thermal treatments under air. The trigonal distortion ( C  3 v pointgroup) of the Fe 3+ octahedral sites, which depends on the synthesis route and thermal treatment, wasinvestigated by X-ray diffraction, Mo¨ssbauer spectroscopy and visible–near infrared (Vis–NIR)spectroscopy. The correlation between diffuse reflectance spectra and structural features of thehaematite samples is reported and discussed herein. The slight increase of the average distortion of theFe 3+ octahedral sites, which depends on the annealing temperature of the precipitated sample, directlylinked to the crystallite size, contrasts with the larger reduction of the sites distortion for the compoundprepared by hydrothermal route due to the occurrence of hydroxyl groups substituted for O 2 À anions aswell as Fe 3+ cationic vacancies. On a local point of view, as shown by Mo¨ssbauer spectroscopy, the Fe 3+ octahedral sites distortion decreases from the centre towards the surface of the grains. Then the smallerthe grain size, the lower the average site distortion. Finally, the reduction of the octahedral distortionwas directly correlated to the two Fe–O charge transfer bands in the visible range and the colour of as-prepared haematites. & 2008 Elsevier Inc. All rights reserved. 1. Introduction a -Fe 2 O 3 is an inorganic red pigment largely used for severalindustrial applications, for instance colouring paints, plastics andenamels, thanks to its low price, low toxicity, and high thermaland chemical stability[1,2]. Nevertheless, the dark-red colour of this mineral can strongly depend on precursors or synthesisroutes[3]. In iron (III)-rich oxides such as haematite or spinels, theintense reddish-brown colour is due to an almost total absorptionof the high-energy region of the visible spectrum [400–550nm]and due to an important reflectivity in the low-energy part of thevisible spectrum [550–800nm][4–6]. In a previous paper[7], visible–near infrared (Vis-NIR) absorption properties of haematiteand spinel ferrites (AFe 2 O 4 ) were correlated to their structuralparameters. Even though numerous authors consider that allabsorption bands in ferrites spinel or haematite result from Fe 3+ 3 d crystal field (CF) transitions[5,6,8,9], in our recent study thetwo main absorption edges (band-gaps) occurring in the visiblerange [400–800nm] were attributed to ligand to metal 2  p (O 2 À ) - 3 d (Fe 3+ ) charge transfers. C  3 v trigonal distortion of [FeO 6 ]octahedra in haematite leading to an additional d orbitals splittingis at the srcin of this double charge transfer. Then, it seemedobvious that the octahedral sites distortion is directlylinked totheenergy 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 isto prepare various a -Fe 2 O 3 compounds synthesized via differentroutes and/or with different thermal treatments and to character-ize the octahedral site distortion of the various haematitecompounds.The crystal structures of these haematite samples have beenstudied by powder X-ay diffraction (Rietveld refinement). Thelocal environments of the Fe 3+ cations and the magneticbehaviour have been investigated by Mo¨ssbauer spectroscopy.Finally, the correlation of their Vis–NIR absorption spectra withtheir structural features will be presented. 2. Experimental details  2.1. Preparation a -Fe 2 O 3 compounds were prepared by a precipitation processin basic medium (i) or by hydrothermal route assisted bymicrowave (ii).(i) A 7.2M NH 4 OH solution was added to a 0.5M aqueoussolution of iron nitrate (Fe(NO 3 ) 3 Á 9H 2 O; Aldrich) in order toprecipitate metal ions with hydroxide form Fe(OH) 3 . Accord-ing to the iron Pourbaix diagram, iron hydroxide is stable in alarge pH range, from 4.5 to 10. The working pH is 9.5. The ARTICLE IN PRESS Contents lists available atScienceDirectjournal homepage:www.elsevier.com/locate/jssc  Journal of Solid State Chemistry 0022-4596/$-see front matter & 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jssc.2008.06.049 à Corresponding author. E-mail address: gaudon@icmcb-bordeaux.cnrs.fr (M. Gaudon). Journal of Solid State Chemistry 181 (2008) 2697–2704  brown precipitate was dried overnight at 100 1 C. Then, inorder to obtain the final a -Fe 2 O 3 oxide, a thermal treatment atvarious temperatures (400, 600, 800 1 C) for 6h under air wasperformed.(ii) The microwave-assisted synthesis was performed with amicrowave digestion system (Model MARS 5, CEM Corp.)operating at a frequency of 2.45GHz. The a -Fe 2 O 3 compoundwas prepared from a 0.05M Fe(NO 3 ) 3 Á 9H 2 O solutionpreparedas in the standard precipitation process (50mL), placed in anautoclave and then treated for 2h at 160 1 C. During theprocess, the maximum pressure reached was about 8bars. Theprecipitate powder is directly obtained with a crystallizedhaematite structure. The powder is just dried a few hoursunder primary vacuum at 100 1 C.  2.2. X-ray powder diffraction X-ray diffraction (XRD) measurements were carried out on aPANalytical X’PERT PRO diffractometer, equipped with anX-celarator detector, using Co K  a radiation because of the fluores-cence of Fe created by Cu K  a irradiation. XRD data were recordedwith 2 y steps equal to 0.017 1 . Diffractograms have been refinedwith Rietveld refinement method[10,11]using FULLPROF s soft-ware[12].  2.3. Vis–NIR diffuse reflectance measurements The UV–Vis–NIR diffuse reflectance spectra have been ob-tained using a VARIAN CARY 5000 spectrophotometer equippedwith an integrating sphere coated with polytetrafluoroethylene(PTFE). Measurements were performed at room temperature forwavelengths varying from 200 up to 800nm. HALON was used aswhite reference. L * a * b * colouring space parameters of thedifferent samples have been calculated from diffuse reflectancecurves R ( l ) and from the three relative sensibility curves: ¯  x ð l Þ ; ¯  y ð l Þ and ¯  z  ð l Þ defined by the CIE-1964. In this system, L * isthe lightness axis [black (0) to white (100)], a * is the green ( o 0) tored ( 4 0) axis, and b * is the blue ( o 0) to yellow ( 4 0) axis. Hence,stronger the a * value, better the red pigment for industrialapplications.  2.4. Mo¨ssbauer spectroscopy 57 Fe Mo¨ssbauer measurements were performed at 293K on aconventional constant acceleration spectrometer (HALDER) usingrhodium matrix source. As the samples contain 8mg natural ironper cm 3 , the line broadening due to thickness of samples can beneglected.The spectrum refinement was performed in two steps. Initially,the fitting of Mo¨ssbauer patterns as a series of Lorentzian profilepeaks allowed the calculation of chemical shift ( d ), amplitude andwidth ( G ) of each peak: thus, experimental hyperfine parameterswere determined for the iron octahedral site. Then, spectraanalysis was made in terms of hyperfine field distribution P  ( H  )using the Hesse and Rubartsch method[13]; G and d werefixed at values determined in the first refinement. This method isoften used for disordered compounds with a distribution of various environments characterized by line broadening andpeak shapes. This method was notably used here because of theline broadening observed for the 400 1 C annealed and thehydrothermal-route compounds, leading to a peak shape differingfrom a Lorentzian profile and so characteristic of disorderedcompounds. 3. Results and discussion  3.1. Structural description The haematite phase a -Fe 2 O 3 crystallizes in hexagonal symmetrywith R -3 c  space group related to the corundum-type structure. TheFe 3+ iron cations are distributed with an ordering of 2/3 of theoctahedral sites (12 c  wickoff positions) within the framework of ahexagonal close-packed array of O 2 À ions. The crystallographicnetwork can be described as ‘‘chains’’ of octahedral sites directedalongthe c  -axisandconstitutedby[Fe 2 O 9 ]dimers — twoface-sharingFe 3+ octahedral sites — separated from each other by an emptyoctahedral site (Fig. 1A). Actually, the iron octahedral environmentexhibits a non-centro-symmetric configuration with C  3 v pointsymmetry. This trigonal distortion is induced by the strong Fe–Feelectrostatic repulsion into the face-sharing octahedral sites formingdimers. Without considering the metallic centre, the octahedralanionic cages already exhibit a C  3 v -type distortion linked, on onehand, to the rhombohedral deformation of the unit cell (octahedralsites are flattened along the c  -axis) but also because of the face-sharing octahedral site. Indeed, consequently to this configuration,only the two octahedral triangular faces perpendicular to the c  -axisremain equilateral, but not equivalent: the area of the common faceof two octahedra forming a dimer is smaller than that of the facessharing an empty cationic site. At last, the cation is displaced fromthe geometric centre of the octahedral site along the c  -axis towardsthe large equilateral face of the [FeO 6 ] octahedra forming two sets of O–Fe bond distances: three short bonds between the ligands of thelarge equilateral face and the metallic centre and three long bondsbetween the ligands of the small equilateral face and the metalliccentre(Fig.1B).Hence,thelongerO–Febondsareassociatedwiththestrongest Fe–Fe interaction in order to relax the constraints of thenetwork and allow optimizing the Madelung energy.  3.2. Structural study The structural parameters of each studied compound wereevaluated by powder XRD. Whatever the synthesis route, a purephase has been obtained with the corundum-type structure. TheRietveld refinement plots (experimental, theory and difference) of the ‘‘precipitated route haematites’’ obtained after heat treatmentat 400 and 800 1 C as well as the ‘‘hydrothermal route haematite’’are presented inFigs. 2a–c, respectively, as illustration. Refinedparameters, i.e. cell parameters, atomic positions, occupancies as ARTICLE IN PRESS bShort O-O bondsLong O-Fe bondsLong O-O bondsShort O-Fe bondsO 2- Fe 3+ acabc c1.45 Å1.15 Å Fig. 1. Structural representations of haematite: chains of [FeO 6 ] dimers along the c  -axis (A) in corundum structure. Illustration of the ‘‘small’’ and the ‘‘large’’equilateral triangles and the Fe 3+ displacement from the octahedral site centre (B). N. Pailhe´et al. / Journal of Solid State Chemistry 181 (2008) 2697–2704 2698  well as reliability factors, are reported inTable 1for a commercialhaematite (Aldrich-99+%-n o 31,005-0) as well as for all preparedcompounds, i.e. precipitated route haematites, annealed at 400,600 and 800 1 C, and the hydrothermal route haematite. Therefinement performed on the commercial powder with fixedoccupancies allowed obtaining the isotropic displacement factors(Debye–Waller factors: B ); then fixed B (A˚ 2 ), 0.35A˚ 2 for Fe 3+ and0.74A˚ 2 for O 2 À , were considered in the case of as-preparedcompounds. Coherent domains were evaluated with Sherrerformula from the integral width of the diffraction patterns’ peaks,taking into account the instrument width. It appears that thecrystallite size of each compound is clearly larger than the Bohrradius of semi-conductors ( $ 5nm). Therefore, no quantumconfinement can be considered, and so diffuse reflectance curvescan be directly related to the corundum-type structure. Moreover,O–Fe and O–O (corresponding to large and small equilateraltriangles) bond distances were compiled from the differentstructure refinements and are reported inTable 2.Firstly, the refinement results of the different precipitated route a -Fe 2 O 3 are discussed. Satisfactory reliability factor values havebeen obtained ( R B $ 3.3%) except in the case of the 400 1 C-annealedcompound ( R Bragg 4 6), where the anisotropy of particles have to betaken into account and has been described, in more detail, in aforthcoming paper[14]. Iron occupancies are near to 100% for allthe compositions annealed at 400, 600 and 800 1 C, showing thatFe 3+ ion sites (12 c  ) are fully occupied. No traces of hydroxylOH À groups have been considered in the structure. In each case,refinements resulted in two different bond distances O–Fe ( $ 2.12and $ 1.94A˚) and two different O–O bond distances associated tothe equilateral triangles, reported inTable 2, as a consequence of the C  3 v -type distortion. Finally, a more flattened octahedral sitecorresponds to one larger and one smaller equilateral face and animportant iron displacement. Thus, the octahedral distortion wasdeduced hereafter from the iron and oxygen atomic positions, theoctahedron [FeO 6 ] being all the more regular as the atomicpositions (  x O and z  Fe ) are close to 1/3. According to atomic positionvalues reported inTable 1and illustrated inFig. 3(despite the high value of standard deviations of  x O and z  Fe coordinates for the400 1 C-annealed compound), one can notice in first approximationthat the octahedral site distortion seems directly linked to thecrystallite size, which is directly governed by the temperature of the thermal treatment. The lower the annealing temperature, thelower the distortion of the octahedral sites because of decrease inthe atomic positions, x o and x Fe . It can be concluded that thecrystallite size governs the structural parameters of the haematitesobtainedfrom the precipitation route.Itcanbe supposed thata lowannealing temperature related to small crystallite size does notallow reaching the stable configuration with the higher Madelungenergy. This point will be discussed in the next section related toMo¨ssbauer investigations.Secondly, for the hydrothermal route sample, the c  -cellparameter (13.8024(6)A˚) and x -oxygen position (0.3128(7))values are particularly high, whereas the Fe occupancy seems tobe different and lower than 1 (0.883(6)) compared to theprecipitated route compounds or literature data[15,16]. In thiscase, the iron occupancy value corresponds approximately to 0.9.It seems obvious that this haematite compound exhibits an ironnon-stoichiometry, with OH À groups substituting for O 2 À ions. Inliterature[17,18], it was shown by FTIR investigations thathydroxyl groups OH À can still be present in the haematitestructure until 1000 1 C. OH À ions would occupy anionic positionsin the hexagonal close-packed anionic sub-lattice of haematite. Inorder to keep the electroneutrality, Fe 3+ vacancies would bestabilized into the network. Consequently, the formula of hydro-xyl-haematite could be written as Fe 2 À  x /3 k  x /3 (OH)  x O 3 À  x . Accordingto the refinement results, the formula of this hydrothermal-routecompound would be roughly Fe 1.8 (OH) 0.6 O 2.4 . A thermogravi-metric analysis was performed, an effective weight loss beingrecorded. The two successive losses at about 140 1 C and from 210to 450 1 C show the occurrence of adsorbed water and bulkOH À hydroxyl groups, respectively. Although these weight lossescould not be accurately quantified because of the strongconvolution of the two phenomena, bulk OH À groups have been ARTICLE IN PRESS 20 26 32 38 44 50 56 62 68 74 802  ( ° ) I obs (dot line)/I cal (full line)I obs -I cal Fig. 2. Rietveld refinement plotof 400 1 C fired(a), 800 1 C annealed samples (b) andhydrothermal-route sample (c). The lowest line is the difference between thecalculated and experimental data. Source: Co( K  a 1 / K  a 2 ). N. Pailhe´et al. / Journal of Solid State Chemistry 181 (2008) 2697–2704 2699  graphically estimated at about 4wt%, a value close to the onecalculated from XRD analysis ( $ 4.5wt%).The expansion of the unit cell in c  -direction (13.8024(6)A˚instead of 13.7505(2)A˚) could be explained by the importantcontent of hydroxyl group; indeed, the equilibrium Fe–OH bondlength, with OH À located in tetrahedral coordination, can becalculated from valence bond calculation following Brown andAltermatt law[19]: the calculated value is about 2.27A˚. This bondis so significantly longer than the higher O–Fe bond distance inhaematite (around 2.12A˚). Then the expansion of the unit celloccurs along the c  -axis and not along the a / b -axes, showing therelaxation of the octahedra along the c  -axis is due to the limitationof Fe–Fe interactions and the presence of H + . Thus, the compoundprepared by hydrothermal route is the only case where the c  -parameter is higher than a ( O 3+1) equilibrium value and theoctahedral sites are elongated along the c  -axis. The consequence of the octahedral site elongation is that Fe 3+ cations relax along the c  -axis. The z  Fe coordinate decreases whereas the x O coordinateincreases, both these values tending towards the equilibriumvalue,equal to 1/3. It shows that the area of the small triangle faceincreases whereas the area of the large ones decreases.  3.3. Mo¨ssbauer spectroscopy 57 Fe Mo¨ssbauer measurements were carried out on theprecipitated compounds and on the commercial haematite inorder to follow the local environment of Fe 3+ iron versus particlesize. Experimental and calculated spectra for 400 and 800 1 C ARTICLE IN PRESS  Table 1 Rietveld refinement results of the precipitated (400, 600, 800 1 C), hydrothermal route and commercial a -Fe 2 O 3 Atom Site x y z  Biso Occupancy 400 1 C  a -Fe  2 O  3 (R-3c) Crystallite size ¼ 17(5)nma ¼ 5.03459(31)A˚, c  ¼ 13.7533(13)A˚R  p ¼ 2.27% Rw  p ¼ 2.97% R Bragg  ¼ 6.72%  Fe1 12 c  0 0 0.35496(25) 0.35 0.991(15)O 18 e 0.3088 (17) 0 14 0.74 1Score ¼ 6.2809 (Berar’s formula) 600 1 C  a -Fe  2 O  3 (R-3c) Crystallite size ¼ 31(6)nma ¼ 5.03506(6)A˚, c  ¼ 13.75053(20)A˚R  p ¼ 1.82% Rw  p ¼ 2.30% R Bragg  ¼ 3.31%  Fe1 12 c  0 0 0.35520(4) 0.35 0.994(3)O 18 e 0.3069(3) 0 14 0.74 1Score ¼ 1.9586 (Berar’s formula) 800 1 C  a -Fe  2 O  3 (R-3c) Crystallite size ¼ 58(5)nma ¼ 5.03594(5)A˚, c  ¼ 13.74439(15)A˚R  p ¼ 2.01% Rw  p ¼ 2.57% R Bragg  ¼ 3.26%  Fe1 12 c  0 0 0.35528(5) 0.35 1O 18 e 0.3071(3) 0 14 0.74 1Score ¼ 1.6359 (Berar’s formula) Hydrothermal a -Fe  2 O  3 (R-3c) Crystallite size ¼ 72(5)nma ¼ 5.03576(17)A˚, c  ¼ 13.80238(58)A˚R  p ¼ 1.72% Rw  p ¼ 2.20% R Bragg  ¼ 5.52%  Fe1 12 c  0 0 0.35408(12) 0.35 0.883(6)O 18 e 0.3127(7) 0 14 0.74 1Score ¼ 2.4150 (Berar’s formula) CommercialFe  2 O  3 (R-3c) Crystallite size ¼ 155(5)nma ¼ 5.03582(5)A˚, c  ¼ 13.75026(12)A˚R  p ¼ 3.03% Rw  p ¼ 3.90% R Bragg  ¼ 3.19%  Fe1 12 c  0 0 0.35537(7) 0.35(4) 1O 18 e 0.3073(6) 0 14 0.74(8) 1Score ¼ 2.4220 (Berar’s formula)  Table 2 Average oxygen–iron and oxygen–oxygen distances in hematites a -Fe 2 O 3 Compound 400 1 C 600 1 C 800 1 C Hydrothermal CommercialO–Fe (A˚) 2.122(8) 2.117(4) 2.118 (3) 2.132 (4) 2.120 (4)1.939(4) 1.942(2) 1.941 (1) 1.936 (2) 1.941 (2)O–O (A˚) 3.023(8) 3.029(5) 3.028(5) 3.001(8) 3.027(5)2.686(8) 2.677(5) 2.679(5) 2.728(8) 2.680(5)O–O: edge distances of the two equilateral triangles. 0,3060,30650,3070,30750,3080,30850,3090,30950,310,310510 Cristallite size (nm)   x   O   A   t  o  m   i  c  p  o  s   i   t   i  o  n 0,35470,35480,35490,3550,35510,35520,35530,35540,35550,3556   z   F  e   A   t  o  m   i  c  p  o  s   i   t   i  o  n 30 50 70 90 110 130 150 170 Fig. 3. Illustration of the correlation between the particle diameter of precipitatedand commercial haematites and atomic positions relative to the octahedral sitedistortion. N. Pailhe´et al. / Journal of Solid State Chemistry 181 (2008) 2697–2704 2700
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