Navegando por Autor "Vandenberghe, Robert Emile"
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Item 57Fe Mössbauer effect study of Al-substituted lepidocrocites.(1996) Grave, Eddy De; Costa, Geraldo Magela da; Bowen, L. H.; Schwertmann, U.; Vandenberghe, Robert EmileSeven A|-containing lepidocrocite samples, ~/-Fe,_xAlxOOH, prepared from FeC12/AI(NO3)3 solutions with initial A1/(A1 + Fe) mole ratios C~ of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 tool/ mol, were examined by means of M6ssbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80~ consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing C i are inconclusive as to whether the A1 cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the therrnoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free ~-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of A1 cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with C~ of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present A1 has been incorporated into the structure.Item About the Morin transition in hematite in relation with particle size and aluminum substitution.(2001) Vandenberghe, Robert Emile; San, E. Van; Grave, Eddy De; Costa, Geraldo Magela daThe results of a MSssbauer study of the Morin transition behaviour in three series of hematite and Al-hematite samples are reviewed and discussed. The first two series comprise small-particle hematites and Al-hematites prepared from decomposition of lepidocrocite, whereas in the third series Al-hematites up to the #m range are obtained from co-precipitated oxinates. It is demonstrated that the Morin transition temperature follows quite well the overall properties of the samples such as particle size and Al substitution, while the transition region is rather determined by all kind of distributive effects. A model involving intermediate states is suggested for the Morin transition behaviour in non-ideal hematite.Item Al hematites prepared by homogeneous precipitation of oxinates : material characterization and determination of the Morin transition.(2002) Costa, Geraldo Magela da; San, E. Van; Grave, Eddy De; Vandenberghe, Robert Emile; Barrón, Vidal; Datas, L.Item Characterization of soil-oxide analogues by applied-field Mossbauer spectroscopy.(1998) Grave, Eddy De; Costa, Geraldo Magela da; Bowen, L. H.; Barrero, C. A.; Vandenberghe, Robert EmileIn this contribution, the effect of high external magnetic fields upon the Mössbauer spectra of aluminous _- and -Fe2O3, of aluminous _- and -FeOOH and of aluminous Fe3O4 is reviewed. It is shown that the shapes of these spectra are characteristic of these materials and also to some extent of their crystallinities and Al-for-Fe substitutions. Based upon this evaluation, the potential for application of external-field Mössbauer spectroscopy to soil-related analytical purposes is demonstrated for two soil samples. Limitations of the technique are discussed. Finally, some suggestions for further research in this field are indicated.Item Influence of nonstoichiometry and the presence of maghemite on the Mössbauer spectrum of magnetite.(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bakker, P. M. A. de; Vandenberghe, Robert EmileSeveral samples of large- and small-particle magnetite (Fe304), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mrssbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400*(2, transforms to a mixture of wellcrystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300"C in a controlled 02 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mrssbauer spectra of the decomposition products at T < 200"C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.Item Low-temperature Mossbauer study of heterosite, (Fe, Mn)PO4.(2013) Grave, Eddy De; Costa, Geraldo Magela da; Alboom, Antoine Van; Vandenberghe, Robert EmileThe heterosite phase occurring in a pegmatitic rock sample was characterized by X-ray diffraction, by energy-dispersive X-ray spectroscopy and by Mossbauer spectroscopy. The orthorhombic unit-cell parameters, expressed in A˚ , were found as a = 9.733 (1), b = 5.837 (1) and c = 4.776 (1). The composition was determined to be (Fe0.54Mn0.43Mg0.04)PO4. Mossbauer spectra recorded at temperatures T of 65K and higher consist of two broadened quadrupole doublets. Their isomer shifts ı are both diagnostic for the ferric state. The dominant doublet (∼60% of total area) exhibits an average quadrupole splitting _EQ,av of 1.62mm/s at room temperature, while the weaker broader doublet has _EQ,av = 0.68 mm/s. For temperatures T≤60K the spectra are composed of a broad sextet and a central quadrupole doublet. The doublet persists down to the lowest applied temperature of 17 K. It is concluded that this doublet is due to an Fe-bearing phase other than heterosite and which gives rise to the inner doublet appearing in the spectra recorded at T≥65 K. The broad sextets, attributable to the heterosite phase, were fitted with model-independent hyperfine-field distributions. However, it was consistently experienced that using the common Lorentzian-shaped elementary sextets composing the distribution, could not adequately reproduce the observed line shapes. Instead, the calculations had to be based on the diagonalization of the complete hyperfine-interaction Hamiltonian. This is due to the unusually strong quadrupole interaction. The as-such calculated hyperfine parameters of the heterosite phase at 17K may be summarized as follows: maximum-probability hyperfine field Bhf,m = 473 kOe, isomer shift ıFe = 0.54 mm/s, average quadrupole coupling constant ½e2qQ = 1.50 mm/s, asymmetry parameter of the EFG _ = 0.80, and polar angles of the hyperfine field with respect to the EFGs principal axes frame˝=40◦ and _ =90◦. The temperature variation of the hyperfine field was interpreted in terms of the Bean–Rodbell (BR) model. The BR parameter, _BR, was found to be 0.90, indicating a first-order magnetic transition at TN = 59.7 K. The temperature variation of the isomer shift is explained by the second-order Doppler shift ıSOD. Using the Debye model for the lattice vibrational spectrum for calculating ıSOD, the characteristic Mossbauer temperature _M was found to be 400 K, which is unusually low for a ferric compound.Item Mossbauer characterization of iron oxides and (oxy)hydroxides : the present state of the art.(2000) Vandenberghe, Robert Emile; Barrero, C. A.; Costa, Geraldo Magela da; San, E. Van; Grave, Eddy DeM¨ossbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.Item Mossbauer studies of magnetite and Al-substituted maghemites.(1998) Costa, Geraldo Magela da; Grave, Eddy De; Vandenberghe, Robert EmileThis paper reviews a systematic M¨ossbauer study of maghemite -Fe2O3 and Alsubstituted maghemites -(Fe1yAly)2O3. Three series of samples prepared from different methods and having different morphological characteristics and aluminum contents were investigated. It was found that both the cation distribution and the solubility limit depend on the preparation method, and no general conclusion in that respect could be inferred. From the temperature dependence of the hyperfine fields the exchange integrals could be calculated, and were found as: JAB = 25 K, JAA = 18 K, and JBB = 3 K. The hyperfine fields show a crossing in the vicinity of 300 K, for both substituted and unsubstituted samples, as a result of the relatively strong A–A interaction. The Curie temperatures were found to be in the range of 948–730 K, the lower value referring to the sample containing 22 mole% Al. The influence of maghemite on the M¨ossbauer spectra (MS) of magnetite was explored in some detail. It was demonstrated on the basis of the MS recorded for a variety of reference mixtures, that it is not possible to resolve the ferric A-site components due to maghemite and magnetite, even with the absorber subjected to a strong external field.Item Study of loss-on-ignition anomalies found in ashes from combustion of iron-rich coal.(2010) Vandenberghe, Robert Emile; Resende, Valdirene Gonzaga de; Costa, Geraldo Magela da; Grave, Eddy DeSamples of a fly and a bottom ash, each before and after ignition at 960 _C, have been studied by X-ray diffraction, Mössbauer spectroscopy, carbon analysis, and thermogravimetric analysis with the aim to explain an observed negative loss on ignition. Ashes after ignition contain more maghemite resulting from oxidation of newly formed magnetite. Moreover, the fly ash that already contained magnetite exhibited an increase of hematite after ignition. Hercynite present in both ashes transforms to hematite and magnetite after the ignition. All these oxidation processes are responsible for a weight gain which may compensate the loss due to the burning of the remaining carbon in the ashes. Also, a-Fe is formed after ignition which may have originated from wustite.Item Synthesis and characterisation of some iron oxides by Sol-gel method.(1994) Costa, Geraldo Magela da; Grave, Eddy De; Bakker, P. M. A. de; Vandenberghe, Robert EmileItem Temperature dependence of the hyperfine parameters of maghemite and Al-substituted maghemites.(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Bakker, P. M. A. de; Vandenberghe, Robert EmileSynthetic aluminum-substituted maghemite samples, 7-(AlyFel_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by M6ssbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: 6A---- 6B-- 0.12 mm/s. From the temperature dependence of 6B it was possible to determine the characteristic M6ssbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing A1 content, at least up to 10 mole%. The temperature dependence of the Aand B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB = --25 K; JAA = --18 K and JBB= --3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% A1. The gradual decrease of the saturation value of the A-site hyperfine field with increasing A1 substitution and the constancy of this quantity for the B sites, suggest that the A1 cations occupy the B sites.Item The center shift in the Mössbauer spectra of maghemite and aluminum maghemites.(1994) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Vandenberghe, Robert Emile; Bakker, P. M. A. deSynthetic, relatively well-crystallized aluminum-substituted maghemite samples, y-(Aly. Fe~_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mrssbauer spectroscopy in the range 8 K to 475 K, and also with an external field of 60 kOe at 4.2 K and 275 K. It was found that there are two different converging models for fitting the zero-field spectra of the maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (6) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements it is concluded that there is a constant difference of 0.12 _+ 0.01 mm/s between 6B and 6A, regardless of the A1 content. For the Al-free sample the center shifts are found as: 64 = 0.370 mm/s and 6a = 0.491 mm/s at 4.2 K and 6n = 0.233 mm/s and fib = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both 6A and fib are observed to decrease with increasing AI concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Hefr.A = 575 kOe and Hofr.B = 471 kOe at 4.2 K and He~,A = 562 kOe and H~er.B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with A1 substitution, while for the A site a slight decrease with increasing A1 content was observed.Item Variable-temperature Mössbauer spectroscopic study of nano-sized maghemite and Al-substituted maghemites.(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Bakker, P. M. A. de; Vandenberghe, Robert EmileSynthetic aluminum-substituted maghemite samples, "g-(Fe~ _xAlx)203, have been prepared by thermal decomposition of Al-lepidocrocite (3,-Fe~_xAlxOOH), with x = 0, 0.04, 0.06, 0.14 and 0.18. The particles are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. MSssbauer spectra were collected at 6 K and from 80 K up to 475 K at steps of 25 K. In a wide range of temperatures the spectra of the non-substituted sample consist of a superposition of a broad sextet and a superparamagnetic doublet, whereas for the Al-maghemites this range is much smaller. From the temperature variation of the fractional doublet area two different parameters were defined: the temperature corresponding to a 50/50 doublet-sextet spectrum (T1/2), and the temperature below which the doublet ceases to exist (To). These two parameters (T1/2 and To) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % AI) and to 100 K and 48 K (18 mole % A1), respectively. The average hyperfine fields at 6 K undergo a steep drop in going from the AIfree sample (I:I~f = 506 kOe) to the sample with 4 mole % A1 (I:Ihr = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: AEQ.A = 0.86 --+ 0.04 mm/s and AEo.B = 0.65 _+ 0.04 mm/s for the 4 mole % A1 sample. The characteristic MiJssbauer temperature, determined from the temperature dependence of the average isomer shift, was found to be in the range of 500-600 K.