Emilia Wolska

Nonstoichiometric structures during dehydroxylation of goethite

Thermal dehydration of goethite (a-FeOOH) at low temperature leads to complete transformation into a hematite-like phase before less than one half of the stoichiometric water content in goethite is expelled. The character of the remaining water has not bee explained as yet. The variations of band positions in the infrared spectra and the specific changes in relative integrated intensities of X-ray lines with the increase of dehydration temperature suggested that OH-groups replace oxygen anions in the hematite crystal lattice, while the electroneutrality is preserved by the cation vacancies. Comparison of integrated intensities observed with intensities calculated on this assumption fully confirms that the hematite-like phase is an iron deficient species with a general chemical formula a F e 2 . x / 3 (0H)J t03 . , . The accurate measurements of lattice constants reveal that while the crystallographic c0-axis decreases steadily with progressing thermal dehydroxylation, the a0-axis reaches a minimum at the temperature at which the nonuniform X-ray line broadening disappears. The largest unit cell of hematite phase after the goethite/hematite transition and the smallest one after the disappearance of selected broadening of X-ray diffraction peaks represent two transitional phases with hematite structure, protohematite and hydrohematite, displaying their own structural and compositional characteristics. The smallest unit cell appearing when the crystalline phase still holds about three percent of structurally bound water * To whom correspondence should be addressed. 224 Emilia Wolska and Udo Schwertmann suggests that the stoichiometric hematite is thermodynamically less stable than this intermediate state.

Structural and spectroscopic characteristics of synthetic hydrohaematite

In order to investigate the identity of water-containing haematite, a method of preparation has been elaborated which leads to samples of hydrohaematite which are “structurally pure” and free of amorphous iron hydroxide. Differential thermal analysis by a lack of endothermic effect at 423 to 473 K on the DTA-curves and by a steady fall of the TG curves up to ∼ 1150 K revealed that water in the preparations must be tightly held in the lattice of haematite. Measurements of intensities of X-ray reflections of the (1 0 4) and (0 2 4) planes in relation to the (1 1 3) plane confirmed Fe3+-deficiency in the haematite cationic sublattice brought about by the presence of OH− ions in the anionic sublattice. The infrared spectrum of hydrohaematite, in addition to six bands from Fe-O variations, exhibits three bands from hydroxyl groups. The effect of silicates on the pattern of the infrared spectra of natural and synthetic hydrohaematites and the discrepancies in the infrared spectra of haematites published to date are discussed.

The influence of aluminum on iron oxides; XV, Al-for-Fe substitution in synthetic lepidocrocite

Lepidocrocite samples, γ-FeOOH, containing 0–10 mole % Al-for-Fe substitution were synthesized at 15°C and pH 8 by oxidizing mixed FeCl2-AlCl3 solutions. The unit-cell parameters a, b, and c were measured from step-counted X-ray powder diffractograms using seven lines and Si as an internal standard. With increasing Al substitution from 0 to 10 mole % the unit-cell edge lengths a, b, and c decreased regularly by 0.3, 0.8, and 0.6%, respectively. Furthermore, the crystals became smaller, but gained in thermal stability. Decrease in crystal size parallel to the y axis led to a significant increase of the OH-stretch vibration and a decrease of the out-of-plane OH-bending vibration due to a weakening of the hydrogen bond between the zig-zag layers in the structure.

Relations between the existence of hydroxyl ions in the anionic sublattice of hematite and its infrared and X-ray characteristics

Abstract The transformation of the goethite (α-FeOOH) iattice into hematite (α-Fe2O3) structure has been studied by X-ray powder diffraction and infrared spectroscopy. It is found that the Pbnm→R 3 c phase transition during thermal dehydration at 180–250°C gives access to a cation-deficient compound, Fe1.67(OH)O2, with hematite structure and hydroxyl groups in the anionic sublattice. Further dehydroxylation yields successively hydrohematite (Fe1.83(OH)0.5O2.5) and hematite (Fe2O3). The temperatures of all these transformations are related to the crystallinity of α-iron oxyhydroxide.

The vacancy ordering and distribution of aluminium ions in γ-(Fe, Al)2O3

Abstract The miscibility of γ-Al 2 O 3 and γ-Fe 2 O 3 was investigated by X-ray powder diffraction, infrared spectroscopy and differential thermal analysis. The Al-maghemites were obtained from coprecipitated Al/Fe-hydroxides after successive hydrothermal treatment at 100°C, dehydration at 300°C, reduction with wet hydrogen at 360°C and oxidation by heating in air at 250°C. Measurements of the unit cell parameters of Al-maghemites indicate the limit of aluminium substitution to be at about y =0.2 in γ -Fe 2− y Al y O 3 . The a 0 and c 0 constants of the tetragonalunit cell decrease from 8.34 to 8.31 A and from 25.02 to 24.90 A respectively. Increasing incorporation of Al 3+ ions in the maghemite structure destroys first the vacancy ordering in the “Li positions” and then the randomness spreads over the whole octahedral sublattice of γ-Fe 2 O 3 . Al-substituted maghemites are thermally more stable than the aluminium-free ones and the temperature of the γ-Fe 2 O 3 →α-Fe 2 O 3 phase transformation increases from about 550°C for pure maghemite to 750°C for γ-Fe 1.8 Al 0.2 O 3 . It was found that about 20% of the total amount of aluminium present in the solid solution is located in the tetrahedral positions.

The effect of cationic and anionic substitution on the α-(Al, Fe)2O3 lattice parameters

Abstract Synthetic Al-hematites prepared under hydrothermal conditions from coprecipitated Al/Fe-hydroxides were studied for their crystallochemical characteristics. X-ray analysis indicates that this preparation method leads to distinctive changes in elementary cell dimensions caused by the simultaneous substitution of Fe 3+ and O 2− by Al 3+ and OH − ions in the hematite sublattices. Substantial deviations from the Vegard rule have been observed not only for the solid solutions obtained at low temperatures but also for their thermal derivatives treated at temperatures not high enough to remove OH − structural groups.

Studies on the ordered and disordered aluminium substituted maghemites

Magnetic properties of γ-Fe2-yAlyO3 reveal thedependence of vacancy ordering and Al-ions distribution on the origin of aluminium substituted maghemites. X-ray, infrared, DTA and room-temperature saturation magnetization characteristics of the γ-(Fe, Al)2O3 solid solution series obtained during oxidation of aluminium substituted magnetites have been compared with similar features of Al-maghemites produces by dehydroxylation of aluminium substituted lepidocrocites. It has been found that the limit of formation of solid solution in both cases lies at about y=0.2. Comparison of changes in saturation magnetization with the increase of Al-content in both series suggests that about 20% of aluminium ions and 5% of vacancies are located on tetrahedral sites in the ordered Al-maghemites obtained from Al-magnetites, whereas the total incorporation of aluminium in the octahedral sublattice was found for disordered Al-maghemites stemming from γ-(Fe, Al)OOH.

Die Bildung von kristallinem Böhmit in gemeinsam gefällten AlFe(III)-Hydroxiden unter hydrothermalen Bedingungen

Under mild hydrothermal conditions originally amorphous aluminium hydroxide coprecipitated with iron(III)ions as Al0,5Fe0,5(OH)3 transforms into aluminium hydroxide crystalline phases which are not observed in products of ageing of pure aluminium hydroxide.By X-ray diffraction analysis, IR spectroscopic studies and microscopic observations it has been found that due to the presence of iron(III) hydroxide it is possible to stabilize the trihydoxide of aluminium i.e. bayerite until 130–140°C. Moreover, with increasing temperature well crystallized boehmite is obtained instead of the usually formed pseudoboehmite.Experiments revealed that the formation of crystalline boehmite does not take place as a process of pseudoboehmit crystallites growth but only as a result of bayerite→boehmite transformation.

X-ray powder diffraction and Mössbauer spectroscopic studies on the solubility limits in γ-FeOOH/γ-AIOOH solid solutions

The formation of substitutional solid solutions between the isostructural γ-FeOOH (lepidocrocite) and γ-AlOOH (boehmite) was investigated by X-ray powder diffraction, Mössbauer spectroscopy, electron microscopic and thermal analysis. Samples of γ-(Fe,Al)OOH were obtained at 15 °C and pH 8 by oxidation of mixed FeCl2-Al(NO3)3 solutions. The unit cell parameters decreased with increasing aluminium content from a=0.3072 nm, b=1.253 nm and c=0.3874 nm, to 0.3053, 1.250 and 0.3858 nm, respectively, The temperature of the differential thermal gravimetry maximum increased with aluminium content up to 10 mol% Al. Quadrupole splitting distribution in the room temperature Mössbauer spectra showed a clear dependence on the concentration of substituted aluminium, enabling determination of the γ-(Fe1−xAlx)1−y/3O1−y(OH)1+y solid solution limits at 0⩽ x⩽ 0.1. The cation deficiency in samples responded to y=0.16 over the whole range of solid solutions formed.

The effect of small deviations in Al0.5Fe0.5(OH)3 stoichiometry on the direction of ageing of coprecipitated amorphous AlFe-hydroxides

Donnees sur le vieillissement des gels amorphes dAl x Fe 1−x (OH) 3 , obtenus par coprecipitation